Synthesis and characterization of arsenic-doped cysteine-capped thoria-based nanoparticles

RESEARCH PAPER

Synthesis and characterization of arsenic-dopedcysteine-capped thoria-based nanoparticles

F. J. Pereira • M. T. Dıez • A. J. Aller

Received: 4 March 2013 / Accepted: 22 July 2013

� Springer Science+Business Media Dordrecht 2013

Abstract Thoria materials have been largely used in

the nuclear industry. Nonetheless, fluorescent thoria-

based nanoparticles provide additional properties to be

applied in other fields. Thoria-based nanoparticles,

with and without arsenic and cysteine, were prepared

in 1,2-ethanediol aqueous solutions by a simple

precipitation procedure. The synthesized thoria-based

nanoparticles were characterized by X-ray diffraction

(XRD), transmission electron microscopy (TEM),

scanning electron microscopy (SEM), energy disper-

sive X-ray spectrometry (ED-XRS), Raman spectros-

copy, Fourier transform infrared (FT-IR) spectroscopy

and fluorescence microscopy. The presence of arsenic

and cysteine, as well as the use of a thermal treatment

facilitated fluorescence emission of the thoria-based

nanoparticles. Arsenic-doped and cysteine-capped

thoria-based nanoparticles prepared in 2.5 M 1,2-

ethanediol solutions and treated at 348 K showed

small crystallite sizes and strong fluorescence. How-

ever, thoria nanoparticles subjected to a thermal

treatment at 873 K also produced strong fluorescence

with a very narrow size distribution and much smaller

crystallite sizes, 5 nm being the average size as shown

by XRD and TEM. The XRD data indicated that, even

after doping of arsenic in the crystal lattice of ThO2,

the samples treated at 873 K were phase pure with the

fluorite cubic structure. The Raman and FT-IR spectra

shown the most characteristics vibrational peaks of

cysteine together with other peaks related to the bonds

of this molecule to thoria and arsenic when present.

Keywords Thoria nanoparticles � Synthesis �Characterization � Precipitation procedure

Introduction

Thorium has been largely used as an excellent raw

material to produce nuclear fuel, to be used in the

experimental nuclear power reactors as well as in the

commercial nuclear industry (IAEA 2005; Palamalai

et al. 1994). Together with uranium, thorium is the

only naturally occurring actinoid element with appli-

cations in the age-dating in geology (Moody and Grant

1999). Thorium has also been used as an alloying

element in magnesium alloys to provide high strength

and resistance at high temperatures (Avedesian and

Baker 1999). Similarly, thorium oxide has been

employed as a hardener of some nickel alloys in the

aerospace and metallurgical industry and as a catalyst

in chemical processes, such as oil fractionizing and

sulphuric acid preparation (Wickleder et al. 2006).

However, during last time, thorium consumption

has been ceased due to the high costs for disposal

of thorium wastes. If thorium passes into the

F. J. Pereira � M. T. Dıez � A. J. Aller (&)

Department of Applied Chemistry and Physics,

Area of Analytical Chemistry, Faculty of Biological

and Environmental Sciences, University of Leon,

Campus de Vegazana, s/n, 24071 Leon, Spain

e-mail: [email protected]

123

J Nanopart Res (2013) 15:1895

DOI 10.1007/s11051-013-1895-8

environmental aqueous systems, different hydroxilat-

ed species of thorium(IV) can be formed (Mompean

et al. 2008), which together with the thorium(IV) ion

are usually bound to different types of particle surfaces

(Geibert and Usbeck 2004), such as colloids and/or

solid hydroxides and oxides (Altmaier et al. 2004;

Neck et al. 2003; Bitea et al. 2003; Rothe et al. 2002;

Ansoborlo et al. 2006). Notwithstanding, important

applications of thoria nanoparticles in the nuclear

industry are still in use (IAEA 2005), although

synthesis of thoria nanomaterial has received little

attention (Dash et al. 2002). Therefore, there is a great

interest in the study of nanocrystalline thoria with

minimum carry over of impurity phases during

synthesis. Nanocrystalline thoria shows excellent

sintering behaviour, being excitingly promising to be

envisaged in the fabrication of fast breeder reactors

and advanced heavy water reactors fuel material

(Chandramouli et al. 1999). Nonetheless, alternative

applications of thoria nanoparticles in medicine and

the environment are desirable. Thus, the use of

fluorescent and/or low-emissive radioactive nanopar-

ticles in medicine can provide advantages, because

they are easy to measure and identify, and also allow

their interaction to be followed more accurately. The

primary use of the radioactive nanoparticles was in

radio-pharmaceutics, as a mean of cancer treatment

(Zhang et al. 2010). However, any other practical

property of the radioactive nanoparticles, such as

luminescence, allows their applicability to be largely

improved. Some typical applications of the photolu-

minescent nanoparticles included biological probes,

sensors and optoelectronic devices (Zhang et al. 2010;

Vogler and Kunkely 2001).

The present work focuses for the first time on the

synthesis and characterization of photoluminescent

arsenic-doped and cysteine-capped thoria-based nano-

particles following a simple precipitation procedure.

Phase formation of the nanocrystalline thoria was

studied by X-ray diffraction (XRD), while transmis-

sion electron microscopy (TEM) was used to evaluate

the crystallite size and morphology of the thoria

nanomaterial. Scanning electron microscopy/energy

dispersive X-ray spectrometry (SEM/ED-XRS)

allowed us to know the morphology and the elemental

composition of the thoria nanoparticles. In addition,

Raman spectroscopy provided information about

changes in the optical phonon line shape associated

with the nanocrystalline thoria and together with

Fourier transform infrared (FT-IR) spectroscopy were

used to obtain information about the functional groups

and additional vibrational features associated with the

developed nanocrystalline material. Photolumines-

cence studies were followed by fluorescence

microscopy.

Experimental

Chemicals

Stock solutions of arsenic(III) (2,000 mg L-1) and

thorium(IV) (10,000 mg L-1) were prepared from

As2O3 and Th(NO3)4�5H2O, in 5 M HCl and dis-

tilled, deionised water, respectively. Hydrochloric

acid (d = 1.19, 37.9 % w/w) and 1 M sodium

hydroxide solutions were employed to adjust pH.

All containers and glassware were soaked in 3 M

nitric acid for at least 24 h and rinsed three times

with distilled, deionised water before use. Distilled,

deionised water with a specific resistivity of 18 MXcm, from a Millipore water purifier system, was used

for the preparation of the stock solutions. The

working solutions were prepared immediately prior

to their use by serial dilutions of the stock solutions.

L-cysteine amino acid (Fluka Chemicals, Buchs,

Switzerland), As2O3 (Merck, Darmstadt, Germany),

Th(NO3)4�5H2O (Merck) and 1,2-ethanediol (Sigma/

Aldrich, Dorset, UK), were of analytical reagent

grade.

Procedure

Thoria-based nanoparticles were prepared by the reac-

tion created on mixing Th(IV) (0.02 M) together with

As(III) (10-3–0.05 M) and/or L-cysteine (0.25 M) at pH

8.0. The reaction was carried out in aqueous solution,

degassed with N2 and in the presence of up to 5 M

1,2-ethanediol to favour precipitation of the solid phase.

The precipitated was separated by centrifugation,

washed three times with an aqueous solution at pH 8.0

and dried at 348 K. It was expected that this preparation

procedure would generate pure precipitates, since the

precipitation stage leaves potential impurities in solu-

tion. For the purpose of the solid phase characterization

by XRD, TEM, SEM/ED-XRS, Raman spectroscopy,

FT-IR spectroscopy and fluorescence microscopy, the

solid nanocrystalline thoria samples were dried at

of 12 J Nanopart Res (2013) 15:1895

123

348 K. Other solid thoria samples were also treated at

873 and 1,373 K.

Instrumentation

XRD

A Philips PW1830 (high-voltage generator) powder

X-ray diffractometer with a Philips PW1710/00

(diffractometer controller), working in Bragg–Brent-

ano diffraction geometry and equipped with a sample

spinner, was used to acquire diffractograms. The

current and voltage used were of 30 mA and 40 kV,

respectively. Powdered samples were mounted in the

form of a thin layer on a zero background Si(911)

substrate using Cu(Ka) as incident radiation. The

scattered intensities were recorded in the 2h span of

158–808. Prior to spectral acquisition, the instrument

was properly aligned and checked for its figure of

merit by conducting a run on a-quartz. Joint Com-

mittee on Powder Diffraction Standards (JCPDS)–

International Center for Diffraction Data (ICDD)

sticks were used to carry out spectral indexing

(Swanson et al. 1974). The crystallite size of the

nanocrystalline material was determined by using

Scherrer and Williamson–Hall equations (Cullity

1978; Williamson and Hall 1953). In the present

case, k was assumed to be 0.154,060 nm, which is

the Cu-Ka1 line. Contribution from the Cu-Ka2 line

was minimal under the conditions used, although

some broadening of the peak profile can occur above

408. Consequently, a little smaller crystallite sizes

were theoretically noted. In our particle size analysis,

machine contribution to the broadening was sub-

tracted from the observed full width at half maximum

(FWHM). To estimate the FWHM for the two

overlapped peaks above 70�, a deconvolution func-

tion (Origin software) was used.

TEM

For observation in TEM, the nanopowder specimens

were directly put onto a 200 mesh (3 mm diameter)

copper grid coated with a holey carbon film and

transferred to TEM load lock. The TEM studies were

carried out on a JEOL 1010 electron microscope

operating at 100 kV. With TEM, the lattice imaging as

well as selected area electron diffraction (SAED)

patterns were acquired.

SEM/ED-XRS

Electron microprobe analysis of the nanoparticle

surface was performed in a JEOL scanning electron

microscope (Model JSM-6100), equipped with an

energy dispersive X-ray detecting system (LINK), and

operated under recommended conditions (15 kV

acceleration voltage and 5 nA probe current).

Raman spectroscopy

Raman spectra of the nanocrystalline powders of ThO2

were recorded in the back scattering geometry at room

temperature. All Raman spectra were obtained using a

BWTEK portable Raman spectrometer, i-Raman

model, fitted with a refrigerated CCD detector. Raman

spectroscopy measurements were performed using the

785-nm line laser, CleanLaze model ([300 mW) as

the excitation source; the power level was set nom-

inally at 100 %, but it had to be reduced on several

occasions due to saturation of the detector. The

experimental conditions were 10 s accumulation time

and 1 min acquisition time; spectra were scanned from

150 to 3,300 cm-1 and the results were processed

using the KnowItAll software (BioRad).

FT-IR

Infrared spectra in the 500–4,000 cm-1 region were

recorded on a Perkin Elmer System 2000 Fourier

transform spectrometer (Norwalk, CT, USA) equipped

with an air-cooled deuterium tryglicine sulphate

(DTGS) detector. The attenuated total reflection

(ATR) accessory utilized was a Perkin Elmer in-

compartment HATR ACCY-FLAT (2000), with flat

top-plate fitted with a 25-reflection, 458, 50 mm ZnSe

crystal, allowing simple sampling of solids, polymer

films and powders. Reproducible contact between the

crystal and the sample was ensured by use of a variable

pressure clamp assembly (2000/GX). Prior to each

analysis, a ZnSe background was scanned at 2 cm-1

resolution for each spectrum; 400 scans were coadded.

In an effort to minimize problems from baseline shifts,

the spectra were baseline-corrected and normalized

using the maximum–minimum normalization in the

KnowItAll software (BioRad).

J Nanopart Res (2013) 15:1895 of 12

123

Fluorescence microscopy

Fluorescence imaging was performed on a Nikon

Eclipse C1si model spectral laser scanning confocal

microscope, equipped with three lasers: diode

(408 nm), argon (488 nm) and helium–neon green

(543 nm), three detection channels and one transmit-

ted light detection channel.

Results and discussion

XRD analysis

Figure 1 shows several XRD patterns of the thorium

oxide nanoparticles prepared in the absence and

presence of 1 M 1,2-ethanediol and treated at two

temperatures. The powdered nanomaterial prepared in

the absence (Fig. 1a) or presence (Fig. 1b) of 1 M 1,2-

ethanediol, treated at 348 K, exhibited XRD patterns

which suggests a typically amorphous structure.

Because the very broad peaks were centered at the

main peaks of the (111), (200) and (220) planes of

ThO2 crystalline, the phase can be considered as

amorphous ThO2. Several superimposed small peaks

owing to nitrate ions also appeared in a few XRD

spectra (Fig. 1a) of the ThO2 nanoparticles prepared in

the absence of 1,2-ethanediol. The effect of L-cysteine

(Cyst) on the crystallisation of the ThO2 nanoparticles,

prepared in the presence of 1 M 1,2-ethanediol and

treated at 348 K, was clearly noted (Fig. 1b). Several

additional peaks, some of them marked as C in Fig. 1b,

were superimposed to those generated from the

amorphous ThO2. These new peaks were clearly

identifiable as due to monoclinic L-cysteine (Harding

and Long 1968; Khawas 1971; Swanson et al. 1974).

However, the XRD spectra of the ThO2-nanomaterial

prepared in the presence of 1 M 1,2-ethanediol and

treated at 873 K (Fig. 1c) showed that the ThO2

nanoparticles possessed a fluorite-type structure with a

cubic Fm3m symmetry, where the nanocrystalline

character was clearly revealed (Swanson et al. 1974).

Similar results were obtained after a thermal treatment

at 1,373 K. No peaks from arsenic species were

confirmed. The main dominant peaks of the ThO2

nanoparticles were identified (Fig. 1c; Table 1) at the

2h values reported in bibliography (Whitfield et al.

1966) and indexed to various crystal planes of the

cubic phase ThO2. No peaks from other phases were

detected, which indicates that the ThO2 nanoparticles

were of high purity.

The lattice parameters of ThO2 at high temperature

(873 K), which represent the cubic fluorite structure,

Fig. 1 XRD spectra of several thoria-based nanoparticles

prepared in the absence (a) and presence (b, c) of 1 M 1,2-

ethanediol and treated at 348 K (a, b) and 873 K (c). (The

symbols N in (a) and C in (b) refer to the nitrate and cysteine

(Cyst) lines)


123

Fm3 m symmetry, were estimated through Bragg law

using the following equation,

SinhB ¼k2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

h2 þ k2 þ l2

a2

� �

s

ð1Þ

where k is the wavelength of the incident X-ray; h, k,

l are Miller indices; hB is the diffraction angle and a is

the lattice parameter. The lattice parameter value,

a = 5.5919 A, estimated using Eq. (1), is a very little

smaller than that of the pure dioxide ThO2,

a = 5.5975 A (Swanson et al. 1974). This is probably

due to the presence of defects in the crystalline

structure with some deviation from the ideal stoichi-

ometry relation, O/Th = 2, of the pure thorium

dioxide (Shein and Ivanovskii 2008).

The crystallite size, d, of the ThO2 nanomaterial was

calculated by the Scherrer equation (Cullity 1978),

d ¼ Kkb CoshB

ð2Þ

where b is the radian measure of the peak FWHM,

d refers to the crystallite size in nm, k is the

wavelength of the incident X-ray, K is a constant

(usually 0.92 for spherical particles) and hB as above

indicates to the Bragg angle subtended at maximum

intensity. The calculated average crystallite size of the

ThO2 nanomaterial was about 5 nm (Table 1), but a

few smaller sizes were obtained using the larger angles

(2h). The average crystallite size of the same nano-

material obtained from the slope (Kk/d) of the straight

line in Fig. 2a is slightly smaller (Table 1), probably

due to the compensation of the experimental errors. It

is interesting to note that the crystallite sizes obtained

for the cysteine-capped ThO2 nanoparticles and the

As-doped ThO2 nanoparticles, treated at 873 K, were

of the same order of magnitude. The presence of

higher concentrations (up to 2.5 M) of 1,2-ethanediol

favoured smaller particle sizes.

The average crystallite dimensions determine the

half-peak broadening of the diffraction line profile.

However, the lattice distortion or microstrain, es,

yields also further broadenings, which are not consid-

ered by the Scherrer equation. The microstrain broad-

ening can be expressed according to Stokes and

Wilson (Balzar 1999),

Table 1 Particle size, d,

and average particle size, d,

obtained by Scherrer’s

equation using Eq. (2) and

from the slope of the

straight line in Fig. 2a for

the Th ? Cyst

nanoparticles prepared in

the presence of 2.5 M 1,2-

ethanediol and treated at

873 K

2h (�) Miller

index

d (nm) from

Eq. (2)

d (nm) from

Eq. (2)

d (nm) from the

slope, Fig. 2a

27.59 (111) 5.58 5.09 2.98

31.95 (200) 5.49

45.70 (220) 5.21

54.26 (311) 5.05

56.89 (222) 5.00

66.82 (400) 4.93

73.54 (331) 4.77

75.88 (420) 4.73

Fig. 2 Scherrer (a) and Gauss–Cauchy (b) plots for the

Th ? Cyst nanoparticles prepared in the presence of 2.5 M

1,2-ethanediol


123

b ¼ 4 eS TanhB ð3ÞBoth broadenings, crystalline domain size and

microstrain, are additive phenomena. Consequently,

combining Eqs. 2 and 3, the two broadening effects

can be joined and described by the so-called the

Williamson–Hall (W–H) equation,

b CoshB ¼ Kk=d

� �

þ 4 eS SinhB ð4Þ

This equation usually allocates a better modelling

of the experimental half-peak broadening (Cabrini

et al. 1971). The plot of b CoshB versus SinhB allows

us to obtain microstrain from the slope and the average

crystallite size from the intercept. This linear additiv-

ity assumes a Lorentzian shape for the combination of

both broadening effects (Cauchy–Cauchy deconvolu-

tion). This means that the peak shape could be better

represented by a Cauchy or Lorentzian function

instead of a Gaussian approach. However, three other

cases can also be considered, i.e. Gauss–Gauss,

Gauss–Cauchy or Cauchy–Gauss deconvolution,

depending on the contribution function to each of the

two effects (Table 2). Other modified methods for

diffraction line profile analysis can be found in recent

bibliography (Mittemeijer and Welzel 2008; Scardi

et al. 2004). According to the determination coeffi-

cient, the best fit to the experimental data was found

for the Gauss–Cauchy approximation (r2 = 0.99387)

(Fig. 2b), although, no important differences were

noted from the other W–H equations. The best average

microstrain and the crystallite size values found

with the Gauss–Cauchy approximation were of

4.91 9 10-3 rad and 6.18 nm respectively. Notwith-

standing, it is worth to remark that Scherrer equation

was also very valid (Fig. 2a) with a determination

coefficient of 0.99463, as long as the particle size was

smaller than 6 nm (Table 1). In conclusion, consistent

results were obtained using both Scherrer and W–H

equations, which means that microstrain contribution

to the half-peak broadening was very small. In other

words, variability in shape and size of the crystallites,

as well as possible contribution from the strain

anisotropy shown little effects to the line profile

broadening and consequently on the apparent size of

the prepared ThO2-based nanoparticles (Scardi et al.

2004).

TEM observation

To confirm the average size of the ThO2 nanoparticles,

this material was studied by TEM. Typical TEM

images are depicted in Fig. 3, which revealed the

homogeneity and small sizes of these particles.

Figure 3 shows bright-field TEM micrographs of the

ThO2 ? Cyst ? As nanoparticles prepared in the

presence of 1 M 1,2-ethanediol and treated at 348 K

(Fig. 3a) and at 873 K (Fig. 3b). The average size of

about 5 nm was easily distinguishable in Fig. 3b, in

agreement with the XRD measurements. Thus, the

average TEM values for d well correlated with the W–

H determination and particularly with Scherrer equa-

tion (Table 1). On the other hand, the ThO2-based

nanoparticles showed a typically cubic morphology as

it is clearly viewed for the ThO2 ? As nanoparticles,

prepared in the presence of 1,2-ethanediol and treated

at 873 K (Fig. 3c). Because of a coalescence of

primary particles, particularly occurred with the thoria

nanoparticles treated at 348 K, a few isolated agglom-

erates shown sizes around 100 nm, but the major part

of the agglomerated particle population contained

domain sizes below 50 nm (Fig. 3a). For the ThO2

nanoparticles treated at high temperatures, grain

boundary formation between particles embedded in

agglomerates was also noted.

Table 2 Equations used

for deconvolution of the

size and microstrain effects

Deconvolution type Equation

Cauchy–Cauchy b CoshB ¼ Kk=d

� �

þ 4 eS SinhB

Gauss–Cauchy b2 Cos2hB ¼ K2k2�

d2

� �

þ 4 es b CoshBSinhB

Cauchy–Gauss b CoshB ¼ Kk=d

� �

þ 16 e2s Sin2hB

�

b CoshB

� �

Gauss–Gauss b2 Cos2hB ¼ K2k2�

d2

� �

þ 16 e2s Sin2hB


123

SEM/ED-XRS analysis

Figure 4 displays a representative SEM image

(Fig. 4a) and the ED-XRS spectra of the ThO2

nanomaterial (Fig. 4b, c). The majority of the particles

included in Fig. 4a are embedded in agglomerates.

Compared to the results of XRD, it can be followed

that every ThO2 particle seen from SEM is congeries

of smaller microcrystals. The particles, an agglomer-

ation of nanoparticles, shown in this case are irregular

shapes with average sizes similar to those found by

TEM.

The ED-XRS spectra (Fig. 4b, c) show that the

ThO2 nanoparticles are obviously composed of tho-

rium and oxygen. The oxygen peak particularly

coming from the thorium oxide species, but some

minor amounts could derive from the residue solvent

on the surface of the particles. When the appropriate

chemicals were used in the preparation of the ThO2-

based nanoparticles, the presence of other elements,

such as arsenic and sulphur, was also noticeable.

Figure 4b, c also shows that the thorium peak

decreased in the presence of arsenic and/or cysteine,

according to the following sequence, Th [ Th ?

As [ Th ? Cyst [ Th ? Cyst ? As, in a similar

way as the sulphur/thorium peak ratio increased. This

is clear confirmation of the presence of cysteine and

arsenic on the thoria nanoparticle surface. On the other

hand, the sulphur peak intensity, as due to the presence

of cysteine, was highest in the absence of arsenic

(Fig. 4c), what suggests some interaction between

arsenic and sulphur. In addition, the arsenic peak

grown in the presence of cysteine, indicating favour-

able retention of arsenic by the cysteine-capped thoria-

based nanoparticles.

Raman spectroscopy

Figure 5 shows the Raman spectra of the nanocrys-

talline thoria samples prepared under the following

three conditions: absence of 1,2-ethanediol and treated

at 348 K (Fig. 5a), and presence of 1 M 1,2-ethane-

diol but treated at 348 K (Fig. 5b) and at 873 K

(Fig. 5c). Thoria has fluorite-type structure

(Fm3m space group) and exhibits only one Raman

active optical phonon mode of F2g symmetry as noted

particularly for all ThO2-based nanoparticles treated at

873 K (Fig. 5c). The Raman spectra of the nanocrys-

talline thoria samples treated at 873 K with an average

particle size of *5 nm (estimated from the XRD

analysis) exhibited a broad asymmetric peak around

460 cm-1. It is common that the frequency of the

Fig. 3 TEM micrographs of several thoria-based nanoparticles

[Th ? Cyst ? As (a, b) and Th ? As (c)] prepared in the

presence of 1 M 1,2-ethanediol (a, b, c), and treated at 348 K

(a) and 873 K (b, c). Th ? As nanoparticles showing the typical

cubic morphology (c)


123

optical phonon shifted to lower values for smaller

particle sizes, as due to confinement of optical phonon

within the particles (Arora and Rajalakshmi 2000).

Absence of periodicity beyond the particle dimension

leads to the relaxation of the selection rule of Raman

scattering, where the scattering vector is null. Under

these situations, the Raman spectra also show contri-

butions from the phonons away from the Brillouin-

zone (BZ) centre. The broadening and the shift of the

Raman line have been found to be dependent on the

shape of the phonon dispersion curve (Rajalakshmi

et al. 2000). The very small Raman band at

1,085 cm-1, appeared only for the thoria and

arsenic-doped thoria-based nanoparticles, was

ascribed to the in-plane bending vibrations of the

O–H group.

The Raman spectra of the ThO2-based nanoparti-

cles, treated at 348 K in the absence (Fig. 5a) and

presence (Fig. 5b) of 1 M 1,2-ethanediol, showed a

relatively great number of additional peaks when

L-cysteine was present, particularly for the thoria

nanoparticles prepared in the presence of 1,2-ethane-

diol. In the absence of L-cysteine, the thoria Raman

peak around 461 cm-1 was well-known for the two

following situations, with and without 1,2-ethanediol,

although with minor shifts and broadenings. In the

absence of both 1,2-ethanediol and L-cysteine, a high

peak at 1,070 cm-1 due to the in-plane bending

vibrations of the O–H group was particularly noted for

the ThO2 nanoparticles decreasing strongly when

arsenic and particularly L-cysteine was joined

(Fig. 5a). However, some Raman peaks of L-cysteine

overlapped the thoria peak. The numerous peaks due

to L-cysteine (Fig. 5a, b) confirmed that L-cysteine was

bound to thoria, and to arsenic when present, at a

surface level. The Raman peaks due to L-cysteine were

assigned, but other Raman peaks appeared confirming

the existence of several meta L-cysteine bonds (Ny-

quist and Kagel 1997). Thus, two small peaks at 357

and 400 cm-1 were ascribed to the vibrational modes

of the As–S group. The Raman spectra also showed

several other bands where the sulphur atoms were

involved. Thus, the peak at 155 cm-1 observed in the

Raman spectra of the Th ? Cyst and Th ? Cys-

t ? As nanoparticles was assigned to the SH torsional

mode and particularly to S polymerization, while the

peaks at 318 and 678 cm-1 were ascribed to C–S out-

of-plane and in-plane bending vibrations respectively.

On the other hand, the peak at 499 cm-1, due to the

S–S bond, indicated the presence of cystine, formed as

a result of L-cysteine oxidation.

The Raman spectra of Th ? Cyst and Th ? Cys-

t ? As nanoparticles also showed peaks at 543 and

845 cm-1 ascribed to the carboxylic group rocking

and waging vibrations respectively. The stretching

Fig. 4 SEM micrograph of the Th ? Cyst ? As nanoparticles

treated at 348 K (a) and the ED-XRS spectra of thoria-based

nanoparticles prepared in the absence (b) and presence (c) of

1 M 1,2-ethanediol and treated at 348 K


123

symmetry frequencies of the O–C–O group were

found at 1,385/1,408 cm-1 with some contribution

from the OH group. The presence of the hydroxyl

group was also confirmed by the very small peak

appeared at 3,115 cm-1, mainly noted when arsenic or

L-cysteine alone were joined to thoria. On the other

hand, two small Raman peaks at 1,580 and

1,623 cm-1 were assigned to the NH3? asymmetric

bending, while the peak at 1,340 cm-1 was ascribed to

the NH3? symmetric bending, probably also with

some contribution from the deformation mode. The

NH3? rocking mode give the peak at 1,042 cm-1,

while the NH3? torsional mode appeared at 385 cm-1.

The presence of the NH3? group indicates that

L-cysteine was mainly present in the zwiteronic form.

The CH and CH2 asymmetric stretching vibrations

originated the bands at 2,969 cm-1 and 2953 sh

cm-1, while the CH symmetrical stretching vibrations

were responsible of the peak at 2,916 cm-1. These

results suggest that L-cysteine was bound to thoria

mainly through the carboxylate group, but also to

arsenic by sulphur.

FT-IR spectroscopy

The FT-IR spectra support the results found by Raman

spectroscopy. Figure 6 shows the FT-IR spectra of

various ThO2-based nanoparticles prepared in the

absence (Fig. 6a) and presence (Fig. 6b, c) of 1 M 1,2-

ethanediol. The FT-IR spectra of the thoria nanopar-

ticles in the absence of L-cysteine showed a charac-

teristic Th–O broad absorption band in the range

400–700 cm-1, particularly at 460, 474, 484 and

506 cm-1 (Fig. 6c, inset). The IR peak at 523 cm-1,

ascribed to the As–O stretching vibrations, also

appeared for the Th ? As nanoparticles. The FT-IR

spectra of the ThO2 nanoparticles containing L-

cysteine and obtained in the presence of 1 M 1,2-

ethanediol showed very resolved and more intense

peaks (Fig. 6b). In these spectra, vibrations associated

with L-cysteine were clearly depicted (Nyquist and

Kagel 1997). However, the FT-IR spectra of the

Th ? Cyst and Th ? Cyst ? As samples prepared in

the absence of 1,2-ethanediol (Fig. 6a) showed a broad

absorption doublet in the region 1,200–1,800 cm-1,

whose area decreased with temperature, disappearing

at 873 K; and thus confirming complete transforma-

tion to nanocrystalline thoria. The FT-IR spectra of the

L-cysteine-capped ThO2 nanoparticles prepared in the

presence of 1,2-ethanediol (Fig. 6b) provided a peak at

848 cm-1 due to the C–S stretching. Between 1,690

and 1,540 cm-1, three IR bands (1,655, 1,623

and 1,585 cm-1) with different intensities were

assigned to the asymmetric bending NH3? vibrations

(Fig. 6b). Nonetheless, Fig. 6a, b shows a broad

stretching vibration NH2 band at approximately

Fig. 5 Raman spectra of thoria-based nanoparticles prepared in

the absence (a) and presence (b, c) of 1 M 1,2-ethanediol and

treated at 348 K (a, b) and 873 K (c)


123

3,410–3,390 cm-1, probably also with some contri-

bution from the hydroxyl vibrations. The typical

absorption near 1,748 cm-1, due to the C=O stretching

(Fig. 6b), showed here a strongly decreased and a

bathocromic shift. The region 1,250–1,000 cm-1

showed a complex structure of weak bands with the

highest peaks at 1,100 and 1,200 cm-1, ascribed to the

C–N and C–O(H) stretching vibrations respectively.

Next, in the region 1,450–1,290 cm-1, typical for the

in-plane OH bending together with the C–O stretching

vibrations, four medium- to high-intensity bands

occurred, indicating the presence of carboxylic acid.

In this way, the O–H group was also confirmed by the

IR broad bands appeared at 3,000–3,200 cm-1, prob-

ably related to the hydrogen-bonded OH vibrations,

and around 3,550 cm-1, due to ‘free’ OH stretching

vibrations, particularly in the absence of 1,2-ethane-

diol (Fig. 6a). In the 2,980–2,840 cm-1 range, there

were several egg-shaped, relatively weak CH and CH2

stretching absorption bands, while the presence of CH2

vibration was also noted at 1,489 cm-1. A weak small

absorption band near 2,586 cm-1, ascribed to the SH

stretching mode, suggests the presence of some free

SH groups, particularly noted in the presence of 1,2-

ethanediol (Fig. 6b). Some of the above peaks can also

show minor contribution from the presence of 1,2-

ethanediol (Fig. 6c). Thus, absorption peaks from the

out-of-plane CH bending (880 cm-1), the CH2 wag-

ging and twisting (1,148 cm-1), the in-plane CH2

bending or scissoring (1,459 cm-1), and the CH and

CH2 stretching vibrations (2,856, 2,924 and

2,960 cm-1) were identifiable, particularly for the

Th ? As nanoparticles (Fig. 6c). In addition, a strong

broad band at 3,350 cm-1 ascribed to the hydroxyl

stretching vibrations was clearly noted for the

Th ? As nanoparticles (Fig. 6c).

Fluorescence microscopy

Figure 7 shows several room temperature fluores-

cence images and fluorescence intensities from vari-

ous ThO2 nanoparticles prepared in the absence and

presence of 1,2-ethanediol, treated at 348 and 873 K.

The emitted radiation was recorded at three wave-

lengths channels, 515 nm (blue), 590 nm (green) and

650 nm (red), which strongly close the maximum

emission lines established for the photoluminescence

spectrum of undoped thoria (Claudel et al. 1975). In

general, the emitted fluorescence from the adequate

emission channel was maximum when excitation at

408 nm, less at 488 nm and minimum at 543 nm. The

increased fluorescence emission from the correspond-

ing ThO2 nanoparticles can be explained by the

generation of free carriers and their capture by defects

in the nanocrystalline material. In nominally pure

Fig. 6 FT-IR spectra of thoria-based nanoparticles prepared in

the absence (a) and presence (b, c) of 1 M 1,2-ethanediol and

treated at 348 K (a, b, c)


123

ThO2 nanomaterial, near band-gap excitation with

light at 408 nm produced optical transitions within an

activator centre or centres which has given rise to

absorbance and broad band fluorescence. As no optical

bleaching occurred, the ground state of the activator

centre must lie in or close to the valence band and

electrons would be introduced into the conduction

band, thus generating free holes in the valence band.

The presence of arsenic and cysteine increased the

fluorescence intensity, particularly when 1,2-ethane-

diol was absent (Fig. 7). In addition, the thermal

treatment at 873 K also facilitated the fluorescence

emission, mainly when L-cysteine was present. Incor-

poration of As(III) into the ThO2 lattice and substitu-

tional replacement for Th(IV) would produce a charge

unbalance which can be compensated for by the

production of oxygen vacancies and/or the conversion

of variable valence thorium to a lower state. Electrons

would be trapped at various defect centres and the

holes captured by the As(III) ions, thereby increasing

the fluorescence intensity associated with the trivalent

ion. The Th(IV) ions would act as the recombination

centres capturing an electron and producing Th(III) in

an excited state, which decaying to the ground state

would emit its characteristic radiation. The fluores-

cence mechanism of the Cyst-capped ThO2-nanopar-

ticles would probably be based on synergistic covalent

and supramolecular interactions.

Conclusions

Synthesis of fluorescent arsenic-doped cysteine-

capped thoria-based nanoparticles by a simple precip-

itation procedure was confirmed for the first time. The

procedure described in this work offers a convenient

way for the synthesis in a hydro-alcoholic medium of

nanocrystalline thoria with an average particle size of

5 nm. The presence of arsenic and cysteine on the

thoria nanoparticles was confirmed by several tech-

niques. The XRD spectra of the ThO2 nanoparticles

treated at 873 K were indexed to fluorite phase of

thorium dioxide without any trace of an extra phase.

The nanostructured morphology was also disclosed by

TEM studies. Raman spectroscopic measurements

revealed optical phonon confinement as well as

additional non-BZ phonon modes. This enhanced

vibrational activity suggested increased atomistic

Fig. 7 Fluorescence intensity and fluorescence images,

recorded at three wavelength channels, 515 nm (blue),

590 nm (green) and 650 nm (red) after excitation at 408 nm,

of the thoria (Th, Th ? As and Th ? Cyst)-based nanoparticles

prepared in the absence and presence of 2.5 M 1,2-ethanediol

(ED) and treated at 348 and 873 K. (Color figure online)


123

transport processes in nanocrystalline thoria. Infrared

spectroscopic measurements, showing discretization

of vibrational levels, were observed in the prepared

nanocrystalline thoria. Visible fluorescence of the

thoria nanoparticles was enhanced by cysteine cap-

ping and arsenic doping.

Acknowledgments A Contract from the Junta de Castilla y

Leon, Consejerıa de Cultura, and Fondo Social Europeo was

awarded to one of the authors (FJP) and is gratefully acknowledged.

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http://dx.doi.org/10.2147/NSA.S7462

http://dx.doi.org/10.2147/NSA.S7462

Synthesis and characterization of arsenic-doped cysteine-capped thoria-based nanoparticles

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