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Applied Surface Science 317 (2014) 480–485

Contents lists available at ScienceDirect

Applied Surface Science

journa l h om epa ge: www.elsev ier .com/ locate /apsusc

aF3 nanoparticles surface modified with tryptophan and their opticalroperties

natoly Safronikhin ∗, Heinrich Ehrlich, Georgy Lisichkinepartment of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia

r t i c l e i n f o

rticle history:eceived 24 April 2014eceived in revised form 25 July 2014ccepted 12 August 2014vailable online 29 August 2014

a b s t r a c t

LaF3 nanoparticles were synthesized by the double-jet precipitation technique in presence of tryptophan(Trp). The product was investigated by TEM, IR, absorption, and luminescence spectroscopies. Interactionof Trp with the nanoparticles results in formation of complexes between Trp and La3+ ions on the nanopar-ticle surface. Surface density of Trp was found as 0.7 molecule nm−2. It is shown that the modifier effectson LaF3 nanoparticle growth and stability of the surface modified LaF3 colloids. Luminescent propertiesof LaF3 nanoparticles modified with Trp (Trp@LaF3) are investigated. It is determined that Trp@LaF3 and

eywords:ryptophananthanum fluorideurface modificationanoparticlesuminescence

Trp have the same profiles of excitation and photoluminescence spectra. Effects of pH, ionic strength,and Trp concentration on luminescence intensity are studied. At the same Trp amounts in the systems,Trp@LaF3 luminescence intensity is about 6 times less than Trp luminescence intensity. Such productscan be used as luminescent labels.

© 2014 Elsevier B.V. All rights reserved.

urface complexes

. Introduction

Surface modification is a way to regulate properties of materialshrough change of surface layer composition and structure. Chemi-al modification involves bonding of a modifier (usually an organicompound) to a surface. This approach is widely used to controliophilicity/liophobicity of a material, particle size and form (at

odification during nanoparticle synthesis, in situ), and nanopar-icle stability. Modifier molecules can induce other properties ofurface modified materials. One of the properties is luminescence.t can be of interest for preparation of functional materials on theasis of rare earth (RE) compounds. Because of their luminescent,agnetic, and other properties, nanosized RE-containing materials

nd applications in medicine and bioassay as luminescent labels foriological imaging [1–3], as contrast agents for magnetic resonance

maging (MRI) [2–6], and as agents for radiotherapy [7,8], photo-ynamic therapy [3,9,10], and neutron capture therapy of cancer11,12]. Now it is of importance to create multifunctional materi-ls on the basis of the RE compound nanoparticles which can bepplied, for example, as multiplexed imaging agents.

In this paper, tryptophan (Trp) is chosen as a modifier. Aminocids are already used as modifiers in the works [7,13–15]. Trp canmprove biocompatibility of the nanoparticles. On the other hand,

∗ Corresponding author at: 119991, Russia, Moscow, Leninskie Gory, MSU, Build-ng 3, Department of Chemistry. Tel.: +7 495 9393666; fax: +7 495 9328846.

E-mail address: safronikhin@yandex.ru (A. Safronikhin).

ttp://dx.doi.org/10.1016/j.apsusc.2014.08.130169-4332/© 2014 Elsevier B.V. All rights reserved.

Trp luminescence is well-known [16,17]. The principal aims of thiswork are to establish how binding of Trp to the surface influencesTrp luminescence and whether Trp binded can use as a luminescentlabel.

Among different RE compounds, lanthanum trifluoride (LaF3) ischosen as a model matrix because it shows chemical stability andlow vibrational energy (∼350 cm−1) which minimizes nonradiativephonon-assisted quenching of the excited states of the dopant ions[18]. For this reason, LaF3 is considered as a good host material forphosphors. A lot of works of last decade describes application ofLaF3 as a matrix to prepare luminescent materials. It is importantthat LaF3 does not luminesce. It has allowed investigating pure Trpluminescence in our work.

So, Trp was employed as a surface modifier at synthesis of LaF3nanoparticles by the double-jet precipitation technique [19,20].The structure and morphology of the synthesized nanoparticlesand stability of their colloid solution were investigated. We paidspecial attention to interaction of the modifier with the nanopar-ticle surface. And finally, we studied luminescent properties of Trpmolecules binded to the surface and compared them with lumines-cent properties of the free amino acid.

2. Experiment

2.1. Materials

All chemicals including lanthanum(III) chloride heptahydrate,sodium fluoride, L-tryptophan, sodium hydroxide, sulfuric acid and

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ficult to make unambiguous conclusion about chemical bondingof Trp to the nanoparticle surface, but we can refer to [28] wheresurface complex formation by interaction between Trp and ZnOnanoparticles was proved. For LaF3 nanoparticles, formation of

Table 1Interplanar spacings in the lattice of synthesized nanocrystals.

hkl d/n (Å)

Trp@LaF3 LaF3 LaF3 [22]

110 3.56 – 3.593111 3.22 3.23 3.228113 2.03 2.00 2.025221 1.75 1.71 1.745411 – – 1.3354224 1.29 1.29 1.2847116 1.16 1.15 1.1597

Table 2Assignment of absorption peaks in IR spectrum of Trp@LaF3.

Absorption peak (cm−1) Assignmenta [23–27]

1654–1580 �ɑsCOO− , �H2O, �NH2

1520–1470 �R, �r1459 �CH2

1430–1350 �CH, �CH2, �sCOO− , �R, �CN (indole)1231 �CH, �R1110–1045 �H(R), �H(r), �CN (aliphatic)863 �H(R)

A. Safronikhin et al. / Applied S

odium chloride were of analytical reagent grade and used withouturther purification. Sodium tetraborate decahydrate was appliedo prepare a borate buffer solution (pH 9.2).

.2. Instruments and characterization

Sizes and morphologies of the synthesized particles were stud-ed by transmission electron microscopy (TEM) on a LEO912 ABMEGA microscope. This equipment was also applied to obtain

elected area electron diffraction (SAED). Chemical compositionf the samples was investigated by infrared (IR) spectroscopy andlemental analysis. IR spectra of the samples in KBr pellets wereecorded on a Thermonicolet Fourier transform IR200 infraredpectrometer. Elemental analysis on carbon was performed on

CHN-2400 analyzer (PerkinElmer) by the frontal chromatog-aphy method. UV–Vis absorption spectra of Trp and Trp@LaF3olutions were measured using UV-1800 Shimadzu spectrome-er and a quartz cuvette (lightpath 10 mm). Photoluminescencepectra of Trp@LaF3 colloids and Trp solutions were collectedn a FLUORAT-02-PANORAMA spectrofluorometer. Measurementsere performed in a quartz cuvette (lightpath 10 mm) at 20 ◦C,

nm step, 25 flashes per second, and middle sensibility. The spe-ific surface area Ssp of the dry product was measured on GeminiII (Micromeritics) surface area analyzer. Using Ssp value, surfaceensity � (molecule nm−2) of Trp on the LaF3 particle surface wasalculated as

= n · NA · 1018

Ssp · m

here n (mol) is amount of Trp on the surface of the product of cer-ain mass m (g), NA is the Avogadro constant (6.022 × 1023 mol−1).

.3. Synthesis of Trp@LaF3

NaOH aqueous solution was added to 100 ml aqueous solution of.3063 g (0.0015 mol) Trp to achieve pH = 8.5. The solution obtainedas heated to 75 ◦C. Further, 15 ml solution of 1.8575 g (0.005 mol)

aCl3·7H2O and 15 ml aqueous solution of 0.6300 g (0.015 mol) NaFere simultaneously added dropwise (the so-called double-jet pre-

ipitation technique) into the Trp solution at 75 ◦C and stirring.hen mixing was finished the system was allowed to cool down

esulting in sedimentation of a white precipitate. Separation of therecipitate and addition water to it resulted in formation of a stableolloid solution. The product was washed with water many times.fter each washing the product was separated by centrifugationt 10,000 rpm for 15 min and dissolved in a new dose of water.emoval of Trp excess was under control by UV–Vis absorptionpectroscopy. As-prepared Trp@LaF3 colloid solution was used forurther investigation.

Unmodified LaF3 particles were also synthesized by the sameethod in the absence of Trp.

.4. Determination of Trp concentration in Trp@LaF3 solution

Trp@LaF3 concentration (c, g l−1) in the purified nanoparticlesolution was determined by drying of a certain volume of the solu-ion:

= m

V,

—mass of dry product (g), V—volume of solution (l).Mass content of Trp in the dry product (w, %) was calculated on

he basis of the combustion analysis data. Trp content (C, mol l−1)n the initial Trp@LaF3 solution was calculated as:

= c · w

100 · M,

Science 317 (2014) 480–485 481

M—molecular weight of Trp, 204.23 g mol−1.According to the estimated C(Trp) value, Trp@LaF3 solutions

with different Trp contents were prepared by diluting.

3. Results and discussion

Synthesis of LaF3 by the double-jet precipitation techniquein the presence of Trp resulted in white precipitate formation.Obviously, coagulation of the product particles is caused by salting-out effect due to appearance of a large amount of NaCl as acoproduct. This effect allowed separating the product that was fur-ther dispersed by adding pure water and formed a stable colloidsolution.

TEM images of Trp@LaF3 and LaF3 nanoparticles synthesizedshow differences between the samples (Fig. 1). Trp@LaF3 consistsof separate nanoplates with an average diameter of 20 nm, thick-ness of 3–4 nm, and a narrow particle size distribution (Fig. 1 a andb). Synthesis of LaF3 under the same conditions in absence of Trpproduces disk-shaped particles of 50–200 nm diameters which areaggregates of LaF3 nanoplates (Fig. 1 d and e). It seems that theaggregates are formed by oriented self-assembly of the primarynanoplates and their intergrowth. Some aggregates assemble bybasal planes (0 0 1). The same processes of LaF3 particles formationare described previously [21]. Therefore the amino acid eliminatesLaF3 nanoparticle aggregation and increases stability of nanopar-ticle solutions. Interplanar spacings estimated from SAED patterns(Fig. 1 c and f) indicate hexagonal LaF3 phase formation for bothsamples (Table 1).

Trp presence in the modified product is approved by IR spec-troscopy (Fig. 2). The IR spectrum of Trp@LaF3 (Fig. 2, spectrum2) has characteristic absorption peaks of Trp rings, carboxyl andamino group (Fig. 2, spectrum 3 and Table 2) which are absentin the IR spectrum of LaF3 (Fig. 2, spectrum 1). Some peaks arenot resolved that is seemingly caused by a small amount of Trpin the product (only on the particle surface). So it is rather dif-

746 �H(R)

a �—stretching vibrations, �—bending and deformation vibrations,as—asymmetric vibrations, s—symmetric vibrations, R—benzene ring, r—pyrrolering.

482 A. Safronikhin et al. / Applied Surface Science 317 (2014) 480–485

nthesized and SAED patterns of these particles (c and f, respectively).

t[1snpsg�n

Table 3Characteristics of Trp@LaF3 nanoparticles and their solution.

Parameter Value

c(Trp@LaF3) (g l−1) 3.6% (in dry powder; combustion analysis data) 0.625Ssp (m2 g−1) 125

−2

Fig. 1. TEM images of Trp@LaF3 (a,b) and LaF3 (d,e) particles sy

he surface complexes is described for citric acid [13,29], glycine13], 4-(2-pyridylazo)resorcinol [30,31], 1,3-diketones [32], and,10-phenanthroline [32]. Strong chemical bonding of Trp to theurface is supported by the fact that it is not eliminated afterumerous washings. Trp complexes can be formed with partici-ation of carboxyl [28,33], amino group [34–36], and �-electron

ystem of indole [27,37,38]. Coordination of Trp through carboxylroup seems more possible in surface complexes, with protonated-amino group oriented to the solution and thus stabilizing theanoparticle.

Fig. 2. IR spectra of LaF3 (1), Trp@LaF3 (2), and Trp (3) in KBr pellets.

�(Trp) (molecule nm ) 0.75

C(Trp in dry powder) (mol g−1) 4.73 × 10−

C(Trp in Trp@LaF3 solution) (mol l−1) 1.7 × 10−4

For characterization of the purified Trp@LaF3 colloid, thenanoparticles were isolated, dried, and analyzed. The results arepresented in Table 3. As followed, the synthesis method used allowsobtaining stable colloid solutions with relatively high concentra-tion of LaF3 (up to several grams per litre).

UV–Vis absorption spectra of Trp@LaF3 and Trp solutions havethe same structure with three strong absorption peaks at 270, 280,and 289 nm (Fig. 3) which belong to electron transitions in Trp[17,28,35,39]. LaF3 colloid solution does not absorb in the UV regionunder study. It indicates that Trp is responsible for absorbance ofTrp@LaF3 solution and that electronic configuration of Trp is notcrucially changed after bonding to the nanoparticle surface.

Trp@LaF3 and Trp solutions were excited by light with � = 270,280, and 289 nm. The most intensive luminescence was detectedat �ex = 289 nm for both samples. Therefore all further photolumi-nescence (PL) spectra were recorded at �ex = 289 nm. PL spectra ofTrp@LaF3 and Trp solutions have identical broad emission peaksat 320–450 nm (Fig. 4) which are characteristic for Trp-containingcompounds [16,17,28,35,39]. There are no any other peaks exceptRaman signal of water at � = 320 nm in the PL spectrum of Trp@LaF3solution.

The increase of ionic strength of solutions achieved by adding of

certain amounts of NaCl resulted in enhancement of Trp solutionluminescence intensity (no more than 10%) and did not affect lumi-nescence intensity of Trp@LaF3 solution (Fig. 5). As ionic strengthdoes not have dramatic effect on luminescence of both samples

A. Safronikhin et al. / Applied Surface Science 317 (2014) 480–485 483

F

as

dir2elcr

Fs

Fig. 5. Effect of ionic strength on luminescence of Trp (1) and Trp@LaF3 (2) solu-

nescence intensity increases in this concentration range but the

ig. 3. UV–Vis absorption spectra of Trp (1), Trp@LaF3 (2), and LaF3 (3) solutions.

ll further luminescence experiments were carried out at ionictrength I = 0.1 M.

Differences in behaviour of the samples were stated for pHependences of luminescence intensity (Fig. 6). Luminescence

ntensity of the Trp solution increases at pH change from 2 to 3,emains constant in the pH range from 3 to 9, and increases in.5 times at pH change to 11. This behaviour can be explained by

xistence of Trp molecule in different ionic forms at different pHevels [40,41]. In the case of Trp coordinated on the LaF3 nanoparti-le surface, there is no change of luminescence intensity in the pHange from 3 to 11, so the structure of Trp surface complex remains

ig. 4. Photoluminescence spectra of Trp (1), Trp@LaF3 (2), and background (3)olutions. �ex = 289 nm, t = 20 ◦C.

tions. Concentration of Trp in Trp solution is 5 × 10−5 mol l−1, concentration of Trpin Trp@LaF3 solution is 1.7 × 10−6 mol l−1, �ex = 289 nm, �em = 354 nm, t = 20 ◦C, pH6.0.

constant. At pH < 3, the complexes are destructed. Therefore sur-face complexes of Trp are stable at the broad pH range and have aconstant value of luminescence intensity.

On the basis of these data, all further experiments were carriedout at pH 9.2 that was maintained with a borate buffer solution.The buffer solution had no absorption and luminescence in thewavelength range under study.

Trp concentration dependences of luminescence intensity aregiven in Fig. 7. The range of Trp concentrations in the mea-surements was limited by maximum Trp content in Trp@LaF3solution (1.7 × 10−4 mol l−1). As follows from the figure, lumi-

maxima of intensity are not achieved. Trp solution possesses, onaverage, 6 times more intensive luminescence than Trp@LaF3 solu-tion with the same Trp content. The origin of this distinction can

Fig. 6. Effect of pH on luminescence of Trp (1) and Trp@LaF3 (2) solutions. Concen-tration of Trp in Trp solution is 5 × 10−5 mol l−1, concentration of Trp in Trp@LaF3

solution is 3.4 × 10−6 mol l−1, �ex = 289 nm, �em = 354 nm, t = 20 ◦C, I = 0.1 M.

484 A. Safronikhin et al. / Applied Surface Science 317 (2014) 480–485

Fs

bclotcp(mwn

c4eCa

coact

Fig. 9. Linear parts of Trp concentration dependences of photoluminescence inten-sity of Trp (1) and Trp@LaF3 (2) solutions. �ex = 289 nm, �em = 354 nm, t = 20 ◦C,

FT

ig. 7. Effect of Trp concentration on luminescence of Trp (1) and Trp@LaF3 (2)olutions. �ex = 289 nm, �em = 354 nm, t = 20 ◦C, I = 0.1 M, pH 9.2.

e close spacing of Trp molecules coordinated on the nanoparti-le surface that leads to interaction between the molecules anduminescence quenching. As the same amount of Trp in the casef Trp@LaF3 solution is concentrated in less volume (only onhe surface) the effect of concentration quenching of lumines-ence occurs in a greater degree (Fig. 8). There are three possiblerocesses after excitation of Trp@LaF3. They are light emissionin the range of 320–450 nm), energy transfer to a nearby Trp

olecule, and energy transfer to the metal ions. The last processas observed at investigation of molecular Trp complexes lumi-escence [35,42,43].

Starting linear parts of the curves range within Trp con-entration up to 1.5 × 10−6 mol l−1 for Trp solution and up to

× 10−6 mol l−1 for Trp@LaF3 solution (Fig. 9). The latter isquivalent to Trp@LaF3 concentration of 0–84.6 × 10−3 g l−1. At(Trp) > 4 × 10−6 mol l−1, the curve departs from linearity that is

result of luminescence quenching.In conclusion, LaF3 nanoparticles modified with Trp are suc-

essfully synthesized. Trp-assisted synthesis results in formation

f a stable colloid solution containing LaF3 nanoparticles with

narrow particle size distribution. Concentration of LaF3 in theolloid solution is relatively high and achieves several grams ofhe compound per litre of the solution. Coordinated by La3+ ions

ig. 8. Scheme of photoluminescence excitation and emission of free Trp (a) and Trp in srp molecule, 3—energy transfer to La3+ ion.

I = 0.1 M, pH 9.2.

on the nanoparticle surface, the amino acid forms the surfacecomplexes which are not destructed under numerous washingby water and changing pH from 3 to 11. Surface density of theTrp complexes is estimated as 0.7 molecule nm−2. CoordinatedTrp molecules have the same luminescent properties as the freeamino acid; however, at the same Trp amounts, luminescenceintensity of Trp@LaF3 solution is 6 times less than luminescenceintensity of Trp solution. It is obviously caused by concentrationquenching of luminescence due to close spacing of coordinatedTrp molecules on the LaF3 nanoparticle surface. As luminescenceintensity of Trp@LaF3 solution is practically independent of ionicstrength and pH the linear part of the concentration depen-dence of luminescence intensity can be used for determinationof C(Trp) or C(Trp@LaF3) in solutions with unknown concentra-tions.

Due to luminescent properties Trp@LaF3 nanoparticles pre-pared can be applied as a luminescent label for biological imagingincluding multiplexed imaging in the case of additional doping by

3+ 3+ 3+

luminescent ions such as Eu and Tb or paramagnetic Gd ions.Besides, Trp can be used as a surface modifier for different metalsalts nanoparticles to provide luminescent properties.

urface complexes (b). Processes: 1—light emission, 2—energy transfer to a nearby

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cknowledgments

This work was financially supported by Russian Foundation forasic Research (Grants nos. 09-03-00875-a and 12-03-00396-a).

eferences

[1] D.K. Chatterjee, A.J. Rufaihah, Y. Zhang, Upconversion fluorescence imaging ofcells and small animals using lanthanide doped nanocrystals, Biomaterials 29(2008) 937–943.

[2] Y. Wang, L. Ji, B. Zhang, P. Yin, Y. Qiu, D. Song, J. Zhou, Q. Li, Upconvert-ing rare-earth nanoparticles with a paramagnetic lanthanide complex shellfor upconversion fluorescent and magnetic resonance dual-modality imaging,Nanotechnology 24 (2013), 175101 (9 pp.).

[3] L. Cheng, C. Wanga, Z. Liu, Upconversion nanoparticles and their compositenanostructures for biomedical imaging and cancer therapy, Nanoscale 5 (2013)23–37.

[4] F. Evanics, P.R. Diamente, F.C.J.M. van Veggel, G.J. Stanisz, R.S. Prosser, Water-soluble GdF3 and GdF3/LaF3 nanoparticles–physical characterization and NMRrelaxation properties, Chem. Mater. 18 (2006) 2499–2505.

[5] H. Hifumi, S. Yamaoka, A. Tanimoto, D. Citterio, K. Suzuki, Gadolinium-basedhybrid nanoparticles as a positive MR contrast agent, J. Am. Chem. Soc. 128(2006) 15090–15091.

[6] Z. Zhou, Z.-R. Lu, Gadolinium-based contrast agents for magnetic resonancecancer imaging, Wiley Interdiscip. Rev. Nanomed. Nanobiotechnol. 5 (2013)1–18.

[7] H. Xing, X. Zheng, Q. Ren, W. Bu, W. Ge, Q. Xiao, S. Zhang, C. Wei, H. Qu,Z. Wang, Y. Hua, L. Zhou, W. Peng, K. Zhao, J. Shi, Computed tomographyimaging-guided radiotherapy by targeting upconversion nanocubes with sig-nificant imaging and radiosensitization enhancements, Scient. Rep. 3 (2013),http://dx.doi.org/10.1038/srep01751, Article number: 1751.

[8] N.J. Withers, N.N. Glazener, J.B. Plumley, B.A. Akins, A.C. Rivera, N.C. Cook, G.A.Smolyakov, G.S. Timmins, M. Osinski, Locally increased mortality of gamma-irradiated cells in presence of lanthanide-halide nanoparticles, Proc. SPIE 7909(2011) 79090L, http://dx.doi.org/10.1117/12.877029.

[9] Y.I. Park, H.M. Kim, J.H. Kim, K.C. Moon, B. Yoo, K.T. Lee, N. Lee, Y. Choi, W. Park,D. Ling, K. Na, W.K. Moon, S.H. Choi, H.S. Park, S.-Y. Yoon, Y.D. Suh, S.H. Lee, T.Hyeon, Theranostic probe based on lanthanide-doped nanoparticles for simul-taneous in vivo dual-modal imaging and photodynamic therapy, Adv. Mater.24 (2012) 5755–5761.

10] A. Shimoyama, H. Watase, Y. Liu, S.-I. Ogura, Y. Hagiya, K. Takahashi, K. Inoue,T. Tanaka, Y. Murayama, E. Otsuji, A. Ohkubo, H. Yuasa, Access to a novel near-infrared photodynamic therapy through the combined use of 5-aminolevulinicacid and lanthanide nanoparticles, Photodiagn. Photodynam. Ther. 10 (2013)607–614.

11] S. Geninatti-Crich, D. Alberti, I. Szabo, A. Deagostino, A. Toppino, A. Barge, F. Bal-larini, S. Bortolussi, P. Bruschi, N. Protti, S. Stella, S. Altieri, P. Venturello, S. Aime,MRI-guided neutron capture therapy by use of a dual gadolinium/boron agenttarget at tumour cells through upregulated low-density lipoprotein trans-porters, Chem. A Eur. J. 17 (2011) 8479–8486.

12] H.K. Cho, H.-J. Cho, S. Lone, D.-D. Kim, J.H. Yeum, I.W. Cheong, Preparation andcharacterization of MRI-active gadolinium nanocomposite particles for neu-tron capture therapy, J. Mater. Chem. 21 (2011) 15486–15493.

13] A.V. Safronikhin, G.V. Ehrlich, G.V. Lisichkin, Synthesis of lanthanum fluoridenanocrystals and modification of their surface, Russ. J. General Chem. 81 (2011)277–281.

14] X. Yang, X. Dong, J. Wang, G. Liu, Glycine-assisted hydrothermal synthesis ofsingle-crystalline LaF3:Eu3+ hexagonal nanoplates, J. Alloys Compd. 487 (2009)298–303.

15] H. Song, L. Zhou, L. Li, F. Hong, X. Luo, Hydrothermal synthesis of char-acterization and luminescent properties of GdPO4·H2O:Tb3+ nanorods andnanobundles, Mater. Res. Bull. 48 (2013) 5013–5018.

16] I. Weinryb, R.F. Steiner, The luminescence of tryptophan and phenylalaninederivatives, Biochem. 7 (1968) 2488–2495.

17] H. Liu, H. Zhang, B. Jin, Fluorescence of tryptophan in aqueous solution, Spect.Acta Part A: Mol. Biomol. Spect. 106 (2013) 54–59.

18] Q. Wang, Y. You, R.D. Ludescher, Y. Ju, Syntheses of optically efficient(La1−x−yCexTby)F3 nanocrystals via a hydrothermal method, J. Lumin. 130(2010) 1076–1084.

[

Science 317 (2014) 480–485 485

19] J. Stávek, M. Sípek, I. Hirasawa, K. Toyokura, Controlled double-jet precipitationof sparingly soluble salts. A method for the preparation of high added valuematerials, Chem. Mater. 4 (1992) 545–555.

20] A.V. Safronikhin, H.V. Ehrlich, G.V. Lisichkin, Chemical modification of the sur-face of highly dispersed metal salt crystals, Prot. Met. Phys. Chem. Surf. 50(2014) 578–586.

21] Y. Cheng, Y. Wang, Y. Zheng, Y. Qin, Two-step self-assembly of nanodisksinto plate-built cylinders through orientated aggregation, J. Phys. Chem. B 109(2005) 11548–11551.

22] E. Staritzky, L.B. Asprey, Lanthanum trifluoride, LaF3; neodymium trifluoride,NdF3, Anal. Chem. 29 (1957) 856–857.

23] C.C. Wagner, E.J. Baran, Spectroscopic and magnetic behaviour of the copper(II) complex of L-tryptophan, Acta Farm. Bonaerense 23 (2004) 339–342.

24] X. Cao, G. Fischer, Infrared spectral, structural, and conformational studies ofzwitterionic L-tryptophan, J. Phys. Chem. A 103 (1999) 9995–10003.

25] A. Kasarcı, D.A. Köse, G.A. Avcı, E. Avcı, Synthesis and characterization of CoII,NiII , CuII and ZnII cation complexes with tryptophan. Investigation of theirbiological properties, Hacettepe J. Biol. Chem. 41 (2013) 167–177.

26] Y. Min, F. Zheng, Y. Chen, Y. Zhang, The synthesis and characterization ofZn–tryptophan complexes and their luminescence properties, J. Mater. Sci.:Mater. Electron. 23 (2012) 1116–1121.

27] C.R. Carubelli, A.M.G. Massabni, S.R. de A. Leite, Study of the binding of Eu3+ andTb3+ to L-phenylalanine and L-tryptophan, J. Braz. Chem. Soc. 8 (1997) 597–602.

28] G. Mandal, S. Bhattacharya, J. Chowdhury, T. Ganguly, Mode of anchoring ofZnO nanoparticles to molecules having both –COOH and –NH functionalities,J. Mol. Struct. 964 (2010) 9–17.

29] A. Safronikhin, T. Shcherba, H. Ehrlich, G. Lisichkin, Preparation and colloidalbehaviour of surface-modified EuF3, Appl. Surf. Sci. 255 (2009) 7990–7994.

30] A. Safronikhin, H. Ehrlich, T. Shcherba, N. Kuzmina, G. Lisichkin, Formationof complexes on the surface of nanosized europium fluoride, Colloid Surf. A:Physicochem. Eng. Aspect 377 (2011) 367–373.

31] A.V. Safronikhin, H.V. Ehrlich, T.N. Shcherba, G.V. Lisichkin, Surface complex-ation onto nanosized lanthanum fluoride, Russ. Chem. Bull., Int. Ed. 60 (2011)1576–1580.

32] A. Safronikhin, H. Ehrlich, N. Kuzmina, G. Lisichkin, The effect of surface mod-ification on Eu3+ luminescence in EuF3 nanoparticles, Appl. Surf. Sci. (2014),http://dx.doi.org/10.1016/j.apsusc.2014.04.062.

33] C. Kremer, J. Torres, S. Domínguez, A. Mederos, Structure and thermodynamicstability of lanthanide complexes with amino acids and peptides, Coord. Chem.Rev. 249 (2005) 567–590.

34] R.S. Dickins, S. Aime, A.S. Batsanov, A. Beeby, M. Botta, J.I. Bruce, J.A.K. Howard,C.S. Love, D. Parker, R.D. Peacock, H. Puschmann, Structural, luminescence, andNMR studies of the reversible binding of acetate, lactate, citrate, and selectedamino acids to chiral diaqua ytterbium, gadolinium, and europium complexes,J. Am. Chem. Soc. 124 (2002) 12697–12705.

35] Y. Wang, S. Liu, Z. Liu, J. Yang, X. Hu, A L-tryptophan-Cu(II) based fluorescenceturn-on probe for detection of methionine, J. Lumin. 147 (2014) 107–110.

36] Ö. Altun, S. Bilcen, Spectrophotometric investigation of DL-tryptophan in thepresence of Ni(II) or Co(II) ions, Eur. J. Chem. 3 (2012) 411–415.

37] T. Shoeib, K.W.M. Siu, A.C. Hopkinson, Silver ion binding energies of aminoacids: use of theory to assess the validity of experimental silver ion basicitiesobtained from the kinetic method, J. Phys. Chem. A 106 (2002) 6121–6128.

38] U.H. Verkerk, J. Zhao, I.S. Saminathan, J.K.-C. Lau, J. Oomens, A.C. Hopkinson,K.W.M. Siu, Infrared multiple-photon dissociation spectroscopy of tripositiveions: lanthanum−tryptophan complexes, Inorg. Chem. 51 (2012) 4707–4710.

39] Y. Min, H. Xia, S. Gao, Y. Chen, Y. Zhang, Simple method to synthesizeNi–Tryptophan layered hybrid complexes and their luminescence properties,Solid State Sci. 14 (2012) 1040–1044.

40] O.A. Weber, V.L. Simeon, Chelation of some bivalent metal ions by racemic andenantiomeric forms of tyrosine and tryptophan, Biochim. Biophys. Acta 244(1971) 94–102.

41] U. Purushotham, G.N. Sastry, A comprehensive conformational analysis of tryp-tophan, its ionic and dimeric forms, J. Comput. Chem. 35 (2014) 595–610.

42] M.D. Gaye-Seye, J.J. Aaron, Fluorescence quenching studies of tryptophan bytrivalent lanthanide ions in aqueous media. Evidence for a lanthanide-inducedintersystem crossing singlet-to-triplet mechanism, J. Chim. Phys. 96 (1999)

1332–1346.

43] M.F. Warsi, M.A. Afzal, A. Mahmood, S. Javed, A.M. Qureshi, M. Shahid, Shorttail tryptophan-functionalized gold nanoparticles: synthesis, characterizationand their fluorescent quenching behavior, New Horizons Sci. Technol. (NHS&T)1 (2012) 33–35.