In situ functionalization of gadolinium oxide nanoparticles with polyethylene glycol (PEG) by pulsed laser ablation in a liquid medium (PLAL)

The spherical shape growth phenomena of Gd-based nanoparticles produced by using PLAL can be described as follows; (i) when the gadolinium target is ablated by nanosecond laser pulses,[r]

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Original Article

In situ functionalization of gadolinium oxide nanoparticles with

polyethylene glycol (PEG) by pulsed laser ablation in a liquid medium

(PLAL)

Atika Doughertya,b,c,*, Clint Harperc, Ferry Iskandara,d, Idam Arifa, Geoff Doughertyc

a Department of Physics, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Indonesia, Jl Ganesha 10, Bandung 40132, Indonesia

b Department of Physics, Faculty of Science and Technology, Nusa Cendana University, Jl Adi Sucipto, Kupang 85001, Indonesia

c Applied Physics and Medical Imaging, California State University Channel Islands (CSUCI), Camarillo, CA 93012, USA

d Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, Bandung 40132, Indonesia

a r t i c l e i n f o

Article history:

Received 18 July 2018

Received in revised form

22 August 2018

Accepted 24 August 2018

Available online 30 August 2018

Keywords:

Synthesis

Functionalization

Gadolinium oxide

Nanoparticles

Pulsed laser ablation

Biocompatible

a b s t r a c t

Gadolinium oxide (Gd2O3) nanoparticles with paramagnetic properties and biocompatible surfaces are promising materials for bioimaging applications We synthesized in situ pegylated Gd2O3(Gd2O3@PEG) nanoparticles by liquid phase pulsed laser ablation (PLAL) of a gadolinium target in a polyethylene glycol (PEG) liquid medium We characterized their shape and morphology using transmission electron mi-croscopy (TEM), and conﬁrmed their crystalline structure with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX) elemental mapping The magnetic properties of the nanoparticles were characterized by vibrating sample magnetometry (VSM) We have found that the crystalline nanoparticles generated have a spherical shape and a narrow distribution with average diameters of 15.0, 11.6, and 6.0 nm, for PEG concentrations of 0.01, 0.05, and 0.10 mM, respectively We veriﬁed that partially oxidized molecules of PEG are attached to the nanoparticle surface as carboxyl groups An analysis of the magnetization of

Gd2O3@PEG nanoparticles revealed highly paramagnetic properties Consequently, PLAL forms a green synthesis of Gd2O3@PEG, opening up new opportunities for bioimaging applications

© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Gadolinium is a remarkable paramagnetic material due to the

transition metal Gd3þion having seven unpaired electrons with a

spin quantum number, s¼ 7/2, which is the largest value among

the elements in the periodical table[1] The spin number of the

Gd3þ ion results in the highest longitudinal relaxation, making

gadolinium-based materials popular in MRI imaging as T1 MRI

contrast agent; gadolinium-based contrast agents are used in 45%

of all MRI diagnosis imaging[2] The Gd3þion has the highest

applications, and the gadolinium isotope 157Gd has the highest thermal neutron cross section contributing to its use for thermal

materials have the possibility of being developed as multifunc-tional contrast agents

According to the Salomon-Bloemberger-Morgan (SMB) the-ory, the performance of a MRI contrast agent can be improved by modifying the ligand design to provide a large hydration number and longer rotation time[4] During the last decade, there have been studies to improve the sensitivity of MRI contrast agents by chelating magnetic ions to form a chelate complex An alterna-tive strategy is to accumulate a high number of magnetic metal atoms in a nanoparticle form Studies on the fabrication of inorganic gadolinium nanoparticles have shown that they can be easily functionalized with other materials, have the ability to carry large ion magnetic materials in their center, and have reduced molecular tumbling rates, which lead to enhanced relaxation time[5]

* Corresponding author Applied Physics and Medical Imaging, California State

University Channel Islands (CSUCI), Camarillo, CA 93012, USA Fax: þ1 8054378864.

E-mail addresses: atika.ahab@mail.com (A Dougherty), clint.harper@usa.com

(C Harper), ferry.iskander@email.com (F Iskandar), idam.arif@asia.com (I Arif),

geoff.dougherty@csuci.edu (G Dougherty).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.08.003

Journal of Science: Advanced Materials and Devices 3 (2018) 419e427

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Among all inorganic gadolinium based nanoparticles,

gadolin-ium oxide (Gd2O3) nanoparticles are very attractive for such

ap-plications due to their high paramagnetism, which depends on

their size, and low toxicity[6] Based on these properties, pegylated

Gd2O3 (Gd2O3@PEG) nanoparticles have been synthesized using

methods, which provided in situ synthesis of functionalized

Gd2O3nanoparticles[7] However, using these methods the

nano-particle products had large nano diameter sizes which we believe

was due to the slow heating processes and non-uniform

temper-ature distributions

To solve the issue, our group has developed a physical approach

to the chemical method, whereby the thermal decomposition can

be performed using pulsed laser ablation in a liquid environment

(PLAL) to produce pegylated Gd2O3 (Gd2O3@PEG) nanoparticles

PLAL is a top-down technique for nanocolloid fabrication, which

uses a focused short or ultrashort pulsed laser to ablate a solid

target, submerged beneath a liquid[8,9] PLAL was introduced by

Patil et al.[8], where a metastable phase of iron oxide was

syn-thesized by ablating an iron target It was demonstrated to be an

effective, simple, and versatile method for synthesizing

nano-particles The resulting nanoparticles are non-toxic and stable

Another advantage is their independence from chemical

pre-cursors, avoiding the use of toxic substances or by-products that

could possibly impose toxicity or block the surface against further

functionalization[12] Since the resulting nanoparticles are in pure

colloidal solution, there is the opportunity for further nanoscale

manipulations such as biofunctionalization [13,14], for biological

sensing, imaging, and therapeutics

The parameters of the PLAL need to be carefully considered in

order to produce monodisperse nanoparticles The critical

param-eters are the near-infrared (NIR) wavelength, the pulse duration in

nanoseconds, and the irradiance at its mid value[15,16] In

addi-tion, the liquid environment also affects the generated

nano-particles, and the polyol liquid medium serves as a template,

surfactant, and biocompatible nanoparticle layer[11] The use of

polyol as a liquid medium is a template that drives the shape of the

nanoparticles to become spherical[17,18]

For the present study, we carried out PLAL of gadolinium foil in

PEG It should be noted that a synthesis of Gd2O3@PEG

nano-particles using PLAL in a PEG liquid medium has not previously

been reported To our knowledge, to date there have been only

several studies of gadolinium nanoparticles prepared by PLAL

Synthesis of gadolinium oxide nanoparticles using PLAL was

suc-cessfully carried out by using a gadolinium plate ablated with

nanosecond pulses from an Nd: YAG laser and ethanol, water, and

acetone as the liquid medium[19] The gadolinium nanoparticles

had a spherical shape and were monodisperse, and stable against

temperature and high pressure

Cueto et al.[11]successfully synthesized platinum nanoparticles

using laser ablation of a platinum target in PEG solvent They

observed that the nanoparticles had a spherical shape with a

smaller diameter and a sharper size distribution compared to the

nanoparticles produced by laser ablation in water In addition,

Besner et al.[10]found that gold nanoparticles produced by PLAL

using a PEG solution as medium were covered with PEG groups

which are useful in biomedical applications

In this paper, we report on a facile synthesis of a colloidal

solution of Gd2O3@PEG nanoparticles by ablating a gadolinium

target in PEG solvent The effects of PEG on the properties of the

Gd2O3nanoparticles such as morphology, particle size,

crystal-lization, and magnetic properties have been investigated We

also investigated the functionalization mechanism of PEG

mol-ecules on gadolinium

2 Experimental 2.1 Synthesis of Gd2O3@PEG nanoparticles using PLAL

A 99.99% pure gadolinium foil (SigmaeAldrich, Singapore) of size 5 5 5 mm3wasﬁrst polished with sandpaper, cleaned in an ultrasonic bath for 30 min and dried in an air dry oven The Gd target was immersed in liquid PEG and then ablated by a nano-second Nd: YAG laser (l¼ 1064 nm) The laser had an energy of 19.80 mJ/pulse, repetition rate 10 Hz, and pulse duration 9 ns The laser spot at the target surface was kept constant at a diameter of

350mm using a lens with a focal distance of 16.50 cm The target was placed in a stage control, and the depth of the liquid above the target was 2 cm Under these conditions the resultingﬂuence was 21.53 J/cm2 The experiment was conducted using PEG of different concentrations (1 kDa, Merck, 0.01, 0.05, 0.1 mM) to investigate the effects of the concentration of the PEG stabilization agent on the nanoparticle products For each PEG concentration, the ablation procedure was completed in 5 min Immediately after ablation, the optical absorption spectra of the resulting colloids were measured

by a UV visible spectrophotometer (Fig 1) The PLAL products synthesized in 0.01, 0.05, and 0.10 mM PEG are assigned to as

GdO-a, GdO-b, and GdO-c, respectively

2.2 Sample preparation for analysis

A transmission electron microscope (TEM) and high resolution TEM (HRTEM) (JEOL-2100F 200 kV) were used to determine morphology, size distribution, and Selected Area Electron Diffrac-tion (SAED) of nanoparticles For this characterizaDiffrac-tion, the nano-particle suspensions were dispersed by soniﬁcation, spotted onto a carbon-coated Cu grid, and then air-dried at room temperature The electron dispersive X-ray analysis system (EDX 20 kV, Oxford EDS system) attached to a scanning electron microscope (Tescan GAIA 3) was used to collect chemical information on the resulting nanoparticles Powder phase composition, its crystalline structure, lattice parameters, and grain size were determined using X-ray diffraction spectroscopy (XRD, Rigaku Smartlab, Cu-Ka at 40 kV

44 mA) X-ray photoelectron spectroscopy (XPS) spectra were ac-quired on a Kratos Axis Ultra spectrometer with a monochromatic

Al KaX-ray source (1486.6 eV) using a pass energy of 20 eV The photoelectron take-off angle was 90 with respect to the sample plane To provide a precise energy calibration, the XPS binding energies were referenced to the C1s peak at 284.6 eV

Fig 1 Optical absorption spectra of Gd colloid solution produced by laser ablation of gadolinium foil in 0.01 mM PEG The inset shows an Absorption Spectrum Fitting (ASF)

A Dougherty et al / Journal of Science: Advanced Materials and Devices 3 (2018) 419e427 420

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Sample preparation was carried out by transferring a drop of

colloidal solution on to a single crystal silicon substrate for SEM,

EDX, and XPS characterizations and on to a glass substrate for XRD

analysis followed by drying at room temperature

The magnetic properties of the samples were characterized

using a vibrating sample magnetometer (VSM, DynaCool PPMS) A

sample for VSM measurement was prepared by drying the

pre-cipitation products of PLAL at 60C for 12 h

3 Results and discussion

3.1 Band gap, morphology and size

Fig 1shows an illustrative UV-visible spectrum of a Gd colloid

sample prepared by ablating gadolinium foil in 0.01 mM PEG-1000

The Gd colloids formed under laser ablation exhibit featureless

op-tical absorption spectra A small peak at 267 nm in the spectra may

indicate the presence of Gd elements[20] Since the sample was

expected to have semiconductor properties, the band gap energy

(Egap) of the sample was determined using absorption spectrum

ﬁtting (ASF) (inset,Fig 1) derived from the Tauc model[21] By using

the linear region at D¼ 0, extrapolation of the (D/l)2versusl1

curve results in a value for Egapof 5.2 eV (for 0.01 mM PEG) The band

gap was found to be 5.24 eV and 5.38 eV for 0.05 mM and 0.10 mM

PEG-1000 respectively These results are close to the reported value

of the direct band gap for materials made from the oxide nanocolloid

of Gd2O3 [22] The PLAL products have a higher value of Egap

(Egap¼ 3.8 eV) This may be due to the microstructure of the particles

and the presence of functionalization ligand groups in the colloid

samples[23] In general, the band gap reﬂects the degree of

crys-tallinity and particle size[24,25] The increasing values of Egapwith

decreasing sizes of the resulting nanoparticles

The morphology and size of the nanoparticles were

character-ized by TEM The micrographs depicted inFig 2show the spherical

morphology of the particles synthesized by the laser ablation The

characteristic shape obtained is most probably due to the PLAL

process and the phenomena that lead to the spherical shape of

nanoparticles, in the same manner as for nanoparticles obtained

using water medium[26]

The spherical shape growth phenomena of Gd-based

nano-particles produced by using PLAL can be described as follows; (i)

when the gadolinium target is ablated by nanosecond laser pulses,

gadolinium atoms and ions are ejected from the gadolinium target

into PEG the moment the laser energy is absorbed by the Gd target

(ii) The ejected gadolinium active species react with oxygen from

the liquid and form an initial oxidized Gd cluster These clusters in

close vicinity aggregate rapidly to form Gd-based nanoparticles and

simultaneously form a region void of Gd clusters since the clusters

are consumed almost completely resulting in embryonic Gd-based

nanoparticle [27,28] However, the supply of gadolinium atoms

outside the void region causes the particle to grow slowly through

diffusion (iii) By using the PEG as a liquid medium, PEG terminates

the diffusion of Gd clusters in competition with the slow growth of

nanoparticles As a result, the interatomic interaction between the

active surface of Gd nanoparticles and partially oxidized products

of PEG molecules dominates, resulting in PEG-coated Gd

nano-particles Since the Gd nanoparticles are now coated with PEG,

further growth of the nanoparticles is prohibited in any direction,

and the shape of the nanoparticles becomes spherical

To examine the use of PEG as a reducing agent, the

concentra-tion of PEG was gradually increased as the liquid medium in the

synthesis process Statistical analysis of TEM size (Fig 2) indicates

that the Gd nanoparticles, synthesized at 0.01, 0.05, and 0.10 mM

PEG-1000 have a spherical shape with mean feret diameters of 15.0, 11.6, and 6.0 nm and standard deviation of 0.44, 0.69, and 0.28 nm, respectively The majority of the particles were of average size, but a few particles of size>20 nm were also found in the samples From our TEM results for the sample synthesized at 0.10 mM, the mean size distribution of Gd nanoparticles is lower than the Gd2O3 pro-duced by PLAL using only water Thus, PEG at concentrations

0.01 mM has an important inﬂuence on the size of the resulting

Gd nanoparticles This can be explained through the Lifshitz-Sloyzov-Wagner (LWS) theory which implies that the size of the ﬁnal particles in a wet synthesis process is related to the viscosity through r 3f 1/h, where r is the radius of the particle product, and

his the viscosity of the solvent[29] This suggests that by increasing the PEG concentration, the viscosity of PEG increases and the size of the Gd nanoparticles decreases concomitantly Higher PEG con-centrations are probably a factor in terminating the diffusion of Gd

nanoparticles

PLAL is a physical approximation of the thermal decomposition synthesis method Compared to that method, chemical heating of solvents/reaction mixtures is a slow heating process as it involves the thermal conductivity/barriers of various materials involved, such as the reaction vessel itself Moreover, the surface of the re-action vessel in contact with the heating source is always at a higher temperature than the rest of the reaction medium, leading

to a non-uniform distribution of temperatures throughout the reaction mixture In the synthesis of nanomaterials, this can result

in large particle size with a wide distribution as reported when synthesizing pegylated Gd2O3 nanoparticles using the thermal

Gd2O3@PEG nanoparticles had an average diameter of 178 nm In comparison, PLAL synthesis involves a direct transfer of high en-ergy from the NIR laser to the reaction medium, causing rapid and homogenous heating of the reactants, which minimizes thermal gradients and provides uniform nucleation and growth conditions resulting in the formation of nanomaterials with a uniform size distribution[15,30] Moreover, such rapid energy transfer creates non-equilibrium conditions in the reaction mixture, resulting in high instantaneous temperatures locally, thus reducing the reac-tion time and improving the crystallinity of the product[30] In this work, by using PLAL the gadolinium target foil was ablated in PEG liquid medium to produce Gd-based nanocolloids It can be clearly seen in Fig 2, the nanoparticles resulting from PLAL are smaller than 100 nm and have a sharper size distribution, in contrast to the material synthesized by the thermal decomposition method

3.2 Crystallinity and phase

In order to study how nanosecond laser radiation centered at

1064 nm determined the crystallinity and phase of the samples, the colloid samples were deposited on quartz substrates at ambient temperature and pressure and analyzed by X-ray diffraction spec-trometer Fig 3displays the XRD patterns of three samples with three different concentrations of PEG The materials show similar peaks, an intense peak at 2q ¼ 29.47 and low intensity peak

relative to theﬁrst one at 42.12 Theﬁrst peak is associated with the crystalline planeð402Þ, and the second with the plane ð313Þ of monoclinic Gd2O3with C2/m group space (the Miller index and

d inter-planar spacing for monoclinic Gd2O3are included in JCPDS:

42e14650)

The XRD patterns show broadening at the peaks that were attributed to carbon materials from the molecular coating of the particles, i.e polyethylene glycol[31], similar to what was observed

in pegylated Gd O nanocrystals, as reported by S€oderlind et al

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[31], Faucher et al.[32], and Ahab et al.[7]) This indicates that after

being ablated, PEG molecules attach to the ablation product These

results are further conﬁrmed by XPS From the samples GdO-a to

GdO-c there is a loss of crystal order detailed by the shape of the

peaks The peak shapes become progressively more broadened in

intensity However, the number of peaks and the position of each

peak stays the same showing that there is no new crystal system

formed at higher PEG concentrations This increased broadening

around the main peaks of the diffraction spectrum is due to

hy-drocarbon chains of the PEG being higher at higher PEG

concen-trations[33]

Typical HRTEM images of Gd2O3@PEG ablated nanoparticles

ablated at 0.01 mM are shown inFig 4(a).Fig 4(b) shows the ring

patterns of selected area electron diffraction (SAED) which are consistent with theð201Þ, ð202Þ, ð202Þ, and ð402Þ planes of the monoclinic Gd2O3structure This is well supported by our X-ray powder diffraction data results The regularity of the lattice plane in the HRTEM image (Fig 4(b)) clearly indicates that the Gd2O3@PEG nanoparticles are typical of polycrystalline material and show that the sample is well-crystallized.Fig 4(c) shows lattice fringes from

an individual crystal with the space between adjacent planes, d, equal to 0.31 nm corresponding to the (111) crystal plane of the monoclinic phase of Gd2O3 (JCPDS: 42-14650 individual crystal with the space between adjacent planes, d, equal to 0.31 nm cor-responding to the (111) crystal plane of the monoclinic phase of

Gd2O3(JCPDS: 42-14650)

Fig 2 Morphology and size distribution of Gd-based nanoparticles synthesized by PLAL using (a) 0.01 mM, (b) 0.05 mM, and (c) 0.10 mM PEG.

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3.3 Elemental analysis

The SEM micrograph displayed inFig 5(a) shows the spherical

morphology of the particles synthesized by the laser ablation

presence of elements such as gadolinium (Gd), oxygen (O), and

carbon (C) The SEM-EDS line scan and the scan region of a

Gd2O3@PEG nanoparticle surface are shown inFig 5(c) and (d),

revealing that the chemical element contents of Gd, O, and C

increase at the center of the nanoparticle The existence of the carbon element on the nanoparticle conﬁrms that the PEG mole-cules successfully attached to the Gd2O3@PEG surface

The elemental maps of Gd2O3@PEG nanoparticles are presented

inFig 6 Gd, O, and C maps characterize the nanoparticle surface, revealing that our PLAL fabrication route produces functionaliza-tion of PEG to Gd2O3nanoparticle surfaces with a homogeneous distribution It can be seen that the entire particle surface consists

of Gd, O, and C

3.4 Functionalization X-ray photoelectron spectroscopy (XPS) is a surfaceesensitive

nanoparticles In this work, XPS spectra were studied to determine functionalization of the PEG molecules to the Gd2O3nanoparticles and to analyze the Gd oxidation state so as to determine which chemical valence state is responsible for the magnetization prop-erties The preparation of the samples for XPS was carried out by deposition of a droplet of the colloidal sample onto a silica wafer which was dried overnight at ambient temperature and pressure

Fig 7 shows the Gd3d, C1s, and O1s XP spectra of the PLAL samples prepared in 0.01, 0.05, and 0.10 mM of PEG1000 The Gd3d XPS spectra of all the samples have been deconvolved to show the two major components at 1219.85 and 1187.65 eV corresponding to

a spineorbit splitting of 32.20 eV resulting in the 3d3/2 and 3d5/2 energy level for Gd[34] The spectra of samples prepared in 0.01 and 0.10 mM (Fig 7(a), (g)), at higher binding energy components merely broadens and featureless with signiﬁcant satellite peak located near higher BE but at 0.05 mM, the peak is sharp and has

Fig 3 X-ray diffraction patterns of the samples produced by laser ablation of Gd target

in solutions of (a) 0.01 mM, (b) 0.05 mM, and (c) 0.10 mM PEG.

Fig 4 (a) HRTEM micrograph of monoclinic Gd 2 O 3 @PEG nanoparticles ablated at 0.01 mM PEG-1000, (b) selected area electron diffraction (SAED) pattern, (c) an individual

Gd O @PEG nanoparticle, and (d) its fast Fourier transform.

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higher intensity compared to 0.01 and 0.10 mM samples These

peaks may be due to the sample with 0.05 mM PEG containing

more native Gd2O3oxidized state (Table 1)

In order to clarify the functionalization of PEG to the

nano-particle ablation products, a detailed analysis of the O1s and C1s

spectra was undertaken The O1s spectrum of the samples shows

four peaks The peak at 529.82 eV corresponds to the oxygen in the

Gd2O3in agreement with earlier XPS work on Gd2O3[31] The peak

at about 531.97 eV originates from the oxygen in the Gd OH group

in PEG[35], the peak at about 532.67 eV originates from oxygen in

the OC ¼ O group[36], and the peak about 533.35 eV originates

from oxygen in the GdeCO3 group[37]

The deconvolution of the C1s XPS spectra (Fig 7(c), (f), (i)) re-veals the existence of peaks which can be assigned to the following:

CeC and CeH groups at 285.05 eV, CeOH and CeOeO at 286.53 eV,

OeC ¼ O at 288.30 eV,47e49 and GdeCO3 at 289.54 eV[37,38]

An extended analysis of the XPS atomic concentration of Gd2O3 nanoparticle surfaces prepared in different PEG concentrations is shown inTable 1 For the sample prepared in 0.01 mM PEG, the O1s

GdeO spectrum corresponds to the formal (Gd3 þ) oxidation state,

as mentioned earlier, and has an intensity of 2.71% Using 0.05 mM PEG, the oxidation state of Gd2O3is higher due to the shift of the

gradually increases The increasing OeC]O intensity is due to the consumption of oxygen in partially oxidized molecules of PEG from the surface of the nanoparticle when the concentration of PEG was increased[38] This unexpected result can be explained by our TEM results which show that this PEG concentration produced a smaller nanoparticle which induces a higher concentration of the gado-linium oxidized state (7.40% GdeO) on the nanoparticle surface However, when 0.10 mM PEG was used as the liquid medium in the PLAL system, the nanoparticle surfaces absorb more oxygen from the PEG resulting in a reduction of the oxidation state of Gd2O3 (GdeO, 2.81%) Moreover, increasing the PEG concentration resul-ted in increased Gd carbonate as well as reduced Gd hydroxide on the chemical surface of the nanoparticle product

The results of the XPS analysis show PEG molecules function-alized to Gd2O3 nanoparticles have been produced by PLAL in a single step reaction During the ablation process, the high tem-perature partially oxidized the PEG solvent resulting in carboxyl functional groups (eOH and COOe) which attached to the Gd2O3

nanoparticle surface During our PLAL process, the nanoparticle surface had increased contact with water (H2O),eOH, and eCOO-groups of partially oxidized PEG products resulting in Gd hydrox-ides and Gd carbonate in the surface chemistry of the native Gd2O3

nanoparticle product This leads us to conclude that reactive Gd2O3 nanoparticle products interacted with water and oxidized PEG molecules at the very top surface layers with Gd carbonate, Gd hydroxide, and Gd oxide being localized at deeper surface layers

Fig 5 (a) SEM image of Gd 2 O 3 @PEG nanoparticles from the laser ablation of Gd foil in

0.01 mM PEG-1000 and (b) EDX spectrum of the nanoparticles (c) The arrow in Fig.

5(a) indicates the EDX line scan position of a Gd 2 O 3 @PEG nanoparticle and (d) the EDX

line scan using the Gd La, O Ka, and C Kasignals.

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The carboxyl functional group plays an important role in dispersing

the nanoparticles and functionalizing them to other organic

ma-terials Based on this understanding, we propose a surface design of

Gd2O3@PEG nanoparticles synthesized by the PLAL method (Fig 8)

3.5 Magnetic properties

Gd2O3@PEG nanoparticle products at 0.01 (a), 0.05 (b), and 0.10 mM

(c) PEG-1000 were investigated by VSM at room temperature

(300 K) in applied magneticﬁelds ranging from 50 kOe to 50 kOe

(Fig 9) In all cases, as expected, the particles exhibit a

para-magnetic behavior The slope of the MH curves correspond to a

susceptibility,c, of 1.87 104, 1.05 104, and 1.24 104,

respectively, which is greater than the susceptibility of Gd2O3@PEG

nanoparticles (c¼ 8.2 10-5) synthesized by the chemical thermal decomposition method[7]

The larger magnetization of Gd2O3@PEG nanoparticles duced by PLAL is due to the smaller size of the nanoparticles pro-duced by this method At nanometric sizes, the ratio of surface spins to the total number of spins increases resulting in magneti-zation enhancement[39] The smaller magnetization of the GdO-b and GdO-c samples compared with the GdO-a sample, shown in

Fig 9, is due to a diamagnetic layer of PEG molecules forming at increasing PEG concentrations which enhances biocompatibility and functionalization but reduces magnetic properties[40] This smaller magnetization can also be explained by considering a critical size of magnetic nanoparticles (in the range of 1e10 nm) In this size range, a magnetic nanoparticle consists of a single domain When the nanoparticle is reduced in size, its magnetization

Fig 7 Gd3d (left panels; a, d, g), C1s (middle panels; b, e, h), and O1s (right panels; c, f, i) XPS spectra of the pegylated Gd 2 O 3 samples.

Table 1

XPS atomic concentration of pegylated Gd 2 O 3 at different concentrations.

Sample XPS Atomic Concentration (%)

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decreases due to increasingly disordered spins within the layer

thickness[41] Previous studies of magnetic nanoparticles showed

that the surface functionalization of small magnetic nanoparticles

can reduce the layer thickness of disordered spins, thus increasing

magnetization of the GdO-c nanoparticles relative to GdO-b is due

to an increased number of chelating bidendate bonds arising from

strongly covalent interactions between the Gd atoms and O2from

the carboxyl group on the Gd2O3nanoparticle surfaces, which help

to reduce the disordered layer This hypothesis is also conﬁrmed by

our O1s XPS results in which the reduction of OeC]O intensity

suggests that the number of partially oxidized molecules absorbed

concentration

4 Conclusion The Gd2O3 nanoparticles with an appropriate surface modi

method based on thermal decomposition We suggest that the more biocompatibility in the process, the more desirable will be the product for biomedical applications In situ production of

Gd2O3@PEG nanoparticles via laser ablation was accomplished using a pure gadolinium foil target We explored the factors con-trolling the growth of the Gd2O3 nanoparticles and their func-tionalization with PEG used as a liquid environment in the PLAL method Our results suggest that PEG affects the size, morphology, and crystallization of the Gd2O3nanoparticles and that the organic molecules of PEG-1000 successfully attached to the nanoparticle surface as carboxyl groups The Gd2O3@PEG nanoparticles have paramagnetic properties at ambient temperature, and they would serve as a multifunctional biomaterial such as a carrier, for target-ing, or as a contrast agent

Conﬂicts of interest There are no conﬂicts to declare

Acknowledgements The work was partially supported by the PKPI Sandwich-Like

2016 program from the Directorate General of Higher Education and Ministry of Research and Technology, Indonesia, an ITB Research and Innovation Grant 2016, and by Applied Physics, Cal-ifornia State University Channel Islands The authors would to thank Dr Amanda Strom (Material Research Laboratory, University

of California, Santa Barbara) for assisting us with the XRD, XPS, and VSM measurements and providing useful comments related to the interpretation

Fig 8 A surface design of Gd 2 O 3 @PEG nanoparticles synthesized by PLAL in a PEG liquid medium.

Fig 9 Magnetic characterization of Gd 2 O 3 @PEG nanoparticles produced by laser

ablation of Gd target in solution of (a) 0.01 mM, (b) 0.05 mM, and (c) 0.10 mM PEG at

300 K.

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