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Fluorescent Functionalized Mesoporous Silica for
Radioactive Material Extraction
Juan Li
a b
a
a
a
a
, Kake Zhu , Jianying Shang , Donghai Wang , Zimin Nie , prof. Ruisong Guo
a
, Dr. Chongxuan Liu , Zheming Wang , Dr. Xiaolin Li & Dr. Jun Liu
a
a
a
a
Pacific Northwest National Laboratory , Richland , WA , USA
b
b
School of Material Science and Engineering, Tianjin University , Tianjin , P.R. China
Accepted author version posted online: 03 Apr 2012.Published online: 12 Jul 2012.
To cite this article: Juan Li , Kake Zhu , Jianying Shang , Donghai Wang , Zimin Nie , prof. Ruisong Guo , Dr. Chongxuan Liu ,
Zheming Wang , Dr. Xiaolin Li & Dr. Jun Liu (2012) Fluorescent Functionalized Mesoporous Silica for Radioactive Material
Extraction, Separation Science and Technology, 47:10, 1507-1513, DOI: 10.1080/01496395.2012.655833
To link to this article: http://dx.doi.org/10.1080/01496395.2012.655833
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Separation Science and Technology, 47: 1507–1513, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0149-6395 print=1520-5754 online
DOI: 10.1080/01496395.2012.655833
Fluorescent Functionalized Mesoporous Silica
for Radioactive Material Extraction
Juan Li,1,2 Kake Zhu,1 Jianying Shang,1 Donghai Wang,1 Zimin Nie,1
Ruisong Guo,1 Chongxuan Liu,1 Zheming Wang,1 Xiaolin Li,1 and Jun Liu2
1
Pacific Northwest National Laboratory, Richland, WA, USA
School of Material Science and Engineering, Tianjin University, Tianjin, P.R. China
Downloaded by [Pennsylvania State University] at 08:03 31 December 2014
2
Mesoporous silica with covalently bound salicylic acid molecules
incorporated in the structure was synthesized with a one-pot,
co-condensation reaction at room temperature. The as-synthesized
material has a large surface area, uniform particle size, and an
ordered pore structure as determined by characterization with transmission electron microscopy, thermal gravimetric analysis, and infrared spectra, etc. Using the strong fluorescence and metal coordination
capability of salicylic acid, functionalized mesoporous silica (FMS)
was developed to track and extract radionuclide contaminants, such
as uranyl [U(VI)] ions encountered in subsurface environments.
Adsorption measurements showed a strong affinity of the FMS
toward U(VI) with a Kd value of 105 mL/g, which is four orders of
magnitude higher than the adsorption of U(VI) onto most of the sediments in natural environments. The new materials have a potential
for synergistic environmental monitoring and remediation of the
radionuclide U(VI) from contaminated subsurface environments.
Keywords fluorescent functionalization;
radioactive material
mesoporous
silica;
INTRODUCTION
Mesoporous silica materials with a large surface area,
ordered pore structure, and controlled particle size have
been shown to be ideal host or carrier materials for functional molecules or nanoparticles such as organic dyes
(1–7), inorganic semiconducting quantum dots, (8–12) and
bio-molecules (8–14). Therefore, mesoporous silica materials have been applied in a wide variety of different areas
such as catalysis, separation, bio-imaging, and drug delivery
(1–21). Besides non-covalently encapsulating guest materials (encapsulation method) (8–9), chemical functionalization is widely used to turn the chemically inert surface of
mesoporous silica into desired reactive surfaces for various
Received 28 July 2011; accepted 5 January 2012.
Address correspondence to Dr. Jun Liu ([email protected]),
Dr. Chongxuan Liu ([email protected]), Dr. Xiaolin Li
([email protected]), Pacific Northwest National Laboratory,
Richland, WA, USA; and Prof. Ruisong Guo (rsguo@tju.
edu.cn), School of Material Science and Engineering, Tianjin
University, Tianjin, P.R. China.
applications. Consequently there have been significant basic
science efforts to functionalize the surface of mesoporous
silica (1–7) (10–21). The approaches to functionalize
mesoporous silica can be generally divided into
co-condensation (one-pot synthesis) (22–25) and grafting
(post-synthesis modification) (15–17) according to the modification procedure before or after mesoporous silica formation. The choice of the functionalization method depends on
the desired properties for a particular application (19).
In addition to the surface chemistry of the functional
groups, the engineered forms and morphologies of
mesoporous silica are important for enabling the required
performances (20) in applications such as separation. For
example, mesoporous nanoparticles with specific surface
functional groups are of great interest for selectively sequestering various aqueous radionuclide and metal ions from
groundwater and wastewater (26–29). The high ionic selectivity, large sorption site density, fast sorption kinetics, and
stable silicate structure yield a potentially efficient material
for capturing metal and radionuclide ions from contaminated groundwater. At the same time, materials with appropriate particle sizes can be used to monitor the transport of
the material in natural and engineered environments to
assess the fate and transport of contaminants and colloidal
particles in subsurface environments, if optical labels can be
attached to these particles (30–34).
A great deal of effort has recently been devoted to the
controlled synthesis of mesoporous silica with specific
morphologies, particle sizes, and pore structures (35–40).
For example, Cai et al. have reported a route for synthesizing mesoporous silica with various morphologies at the nanometer (average size of 100 nm) and micrometer (average
size of 1 mm) scales by varying the concentration of surfactant and tetraethyl orthosilicate (TEOS) (38). Ordered
mesoporous silica with radially-oriented mesochannels has
been prepared using anionic surfactants (N-lauroylsarcosine
sodium) as templates (39). And the group of Thomas Bein
has synthesized mesoporous silica nanoparticles in the form
of stable colloidal suspensions with 50 to 100 nm particle
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J. LI ET AL.
sizes from concentrated precursor solutions with high yields
by using triethanolamine (TEA) as a substitute for the more
commonly used hydrolysis initiators NaOH or NH4OH
(40). Additionally, a wide range of functionalized mesoporous materials has been studied for selective binding of radioactive elements (20,41,42). However, there has been no
report of a controlled synthesis of fluorescent mesoporous
nanoparticles or their application based on simultaneous
selective binding and optical tracking.
In this paper, we developed a one-pot method for synthesizing fluorescent functionalized mesoporous silica (FMS)
with a uniform particle size and ordered pore structure for
binding and potentially monitoring radionuclides. The
FMS structure and the integration of functional molecules
into the silica host matrix were characterized by transmission
electron microscopy (TEM), thermal gravimetric analysis
(TGA), and infrared (IR) spectra, etc. The strong fluorescence and adsorption affinity toward radionuclide ions such
as U(VI) (Kd value of 105 mL=g) of the FMS were demonstrated by fluorescence and adsorption measurements. We
therefore conclude that FMS can be used as a simultaneous
fluorescent tracking and remediation reagent to extract the
radionuclide U(VI) from contaminated sediments.
EXPERIMENTAL SECTION
Chemicals
All chemicals were used directly without further purification, including salicylic acid (SA, minimum 99.8%, SigmaAldrich), N, N-dimethylformide (DMF, anhydrous, 99.8%,
Sigma-Aldrich), 1,10 -carbonyldiimidazole (CDI, reagent
grade, Aldrich), aminopropyltriethoxysilane (APTES, 99%,
Sigma-Aldrich), ethanol (anhydrous, Sigma Aldrich), cetyltrimethylammonium chloride solution (CTAC, 25 wt%
solution in water, Aldrich), TEOS (Fluka), TEA (minimum
98%, Sigma), and deionized (DI) water. Hydrochloric acid
(HCl, 37%, A.C.S. Reagent, Sigma-Aldrich) and ethanol
were used for template extraction.
Material Synthesis
Fluorescent mesoporous silica nanoparticles were
synthesized by combining Fryxell’s method of activation
of SA (18) and Bein’s synthesis of a colloidal suspension
of nanosized mesoporous silica (40,44,45). FMS was
synthesized by controlling the hydrolysis of TEOS using
organic amine. SA is activated by CDI in DMF solution
and then linked to APTES for further functionalization
of FMS. Detailed synthesis is as below.
Activation of SA by CDI and Binding with APTES
The reaction equation of SA activation is shown in
Fig. 1a. SA (2.76 g, 20 mmol) was dissolved in 50 mL of
DMF. Then CDI (3.243 g, 20 mmol) and 12 mL of DMF
were added to the above solution. This mixture was stirred
FIG. 1. Chemical reactions showing the synthesis of SA incorporated
mesoporous silica. (a) Activation of SA with carbonyldiimidazole (CDI).
(b) Linking SA to aminopropyltriethoxysilane (APTES). (c) Schematic
of SA incorporated into mesoporous silica.
under nitrogen for 1 h until the evolution of CO2 ceased.
Then APTES (4.68 mL, 20 mmol) was added into the
mixture to link with activated SA (Fig. 1b). The final clear
solution was stirred under nitrogen for 16 h and denoted
as APTES-SA.
Synthesis of FMS
Fluorescent mesoporous silica nanoparticles were
prepared with a modified method based on Bein’s approach
(40,44,45). In the typical preparation of fluorescent
mesoporous silica nanoparticles (e.g., molar ratio of
TEOS: TEA ¼ 1:4, reaction time 3 h, and molar ratio of
APTES-SA: TEOS ¼ 1:10), water (6.4 mL), ethanol
(1.05 mL), and CTAC (1.04 mL, 25 wt% CTAC solution)
were mixed and stirred at room temperature for 30 min.
TEA (1.85 g) was subsequently added to the mixture and
the solution was stirred for 1 h until it dissolved. TEOS
(0.692 mL) and APTES-SA (1.05 mL) were also added into
the mixture and stirred for 3 h at room temperature.
Figure 1c shows a cartoon of the synthesized FMS in which
SA molecules were bonded to the wall through a
co-condensation reaction. The amounts of TEA, TEOS,
and APTES-SA as well as the reaction time were varied to
investigate their effects on the properties of particles. In
the following descriptions, the samples were labeled to
reflect the molar ratio of TEOS to TEA, the reaction time,
and the molar ratio of TEOS to APTES-SA. For example,
S4-0.5-0.1 stands for a TEOS: TEA ratio of 1: 4, a reaction
time of 0.5 h, and an APTES-SA:TEOS ratio of 1:10. The
synthesis may also be done using biodegradable and
inexpensive block copolymer templates (46).
Extracting surfactant templates from the fluorescent
mesoporous silica nanoparticles proceeded as follows:
Ethanol (30 mL) was added into the translucent, colloidal
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MESOPOROUS SILICA FOR RADIOACTIVE MATERIAL EXTRACTION
1509
aqueous suspension of the mesoporous materials. The
resulting precipitates were washed by repeated centrifugation at 10,000 rpm for 10 min and then re-dispersed in
ethanol=DI H2O (1:1) under sonication. Concentrated
hydrochloric acid (3 mL) in ethanol (240 mL) was used for
template extraction. Usually, the sample (1.0 g) was stirred
in a 100 mL extraction solution at room temperature for
more than 6 hours. This procedure was repeated three
times. Subsequent washing was performed by repeated
centrifugation at 10,000 rpm for 10 min and re-dispersion
in DI H2O under sonication. All products were suspended
and stored in DI H2O for further characterization and use.
Samples for absorbance and fluorescence measurements
were prepared by diluting stock solutions of the different
samples and then separating the solid from DI H2O by
centrifuging with Amicon Ultra-4 centrifugal filter devices
(low-binding Ultracel membranes [30 K Dalton]) at
5,000 rpm for 10 min. The supernatant was discarded.
Characterization
TEM analysis was performed on a JEOL JSM-2010
TEM microscope operated at 200 kV. The surface area
was determined using Quantachrome Autosorb Automated
Gas Sorption Systems. The Brunauer-Emmett-Teller (BET)
surface area was obtained with the single-point adsorption
method. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted with a
TGA=DSC 1 (Mettler Toledo) thermogravimetric analyzer
in argon at a scan rate of 10 C=min from room temperature
to 800 C. Absorbance and fluorescence measurements were
performed on a Tecan Safire2 Microplate reader system
equipped with ultraviolet visible (UV-Vis) fluorescence
and an absorbance reader (kex ¼ 304 nm, kem ¼ 436 nm).
Uranyl ion adsorption onto the material was measured with
UV-Vis recording spectrophotometry (UV-2501 PC,
Shimadzu Scientific Instruments, Columbia, MD). FTIR
spectra were recorded with a Nicolet iS10 spectrometer
(Thermo Scientific) at a resolution of 1 cm1 or higher,
employing a diamond Smart ITR accessory.
RESULTS AND DISCUSSION
The morphology of the synthesized FMS changes with
the ratio of TEOS:TEA, the reaction time, as well as with
the ratio of APTES-SA:TEOS (Fig. 2). The FMS
(S4-3-0.1) sample obtained under typical conditions (TEOS:
TEA ratio of 1:4, a reaction time of 3 h, and an APTES-SA:
TEOS ratio of 1:10) exhibits a spherical morphology with a
uniform particle size of about 230 nm and a radially aligned,
pore structure (Fig. 2a). The inset in Fig. 2a is a higher
resolution image showing the pores packed in this radial
configuration. The molar ratio of TEOS: TEA was varied
from 1:4 to 1:1 to investigate its effect on the particle size
of fluorescent mesoporous silica. Small mesoporous silica
particles of about 120 nm were obtained by decreasing the
FIG. 2. TEM images of FMS obtained at various conditions. a) FMS
obtained at optimized conditions with the molar ratio of TEA: TEOS
about 4:1 and the molar ratio of APTES-SA: TEOS about 1:10. The reaction time is 3 h. Inset is the enlarged image showing the porous wall. b)
FMS obtained with the molar ratio of TEA: TEOS about 1:1. c) S4-0.50.1 FMS obtained with half an hour reaction. d) S4-24-0.1 FMS obtained
with 24 h reaction. Inset is the zoom in the image of the porous structure.
e) S4-3-0.02 FMS obtained with the molar ratio of APTES-SA: TEOS
about 1:50. f) S4-3-0.4 FMS obtained with a molar ratio of APTES-SA:
TEOS of about 1:2.5.
TEOS: TEA ratio to 1:1 while keeping all other conditions
the same (Fig. 2b). The trend of decreasing particle sizes
with increasing TEOS:TEA ratios is consistent with the
results observed for the pure mesoporous silica nanoparticles synthesized by Möller et al. (38). However, the variation of the amount of TEA did not significantly affect
the pore size of the mesoporous nanoparticles (Fig. 3).
We also studied the influence of the reaction time on the
formation of FMS to study the growth process of the fluorescent mesoporous silica nanoparticles. The TEM image of
S4-0.5-0.1 (Fig. 2c) and S4-24-0.1 (Fig. 2d) show the morphology of the sample obtained after 0.5 h and 24 h of reaction time while all other conditions were kept the same.
Silica particles began to form after the mixture was stirred
at room temperature for 0.5 h, but at this stage no individual spheres could be detected, the formed silica particles
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1510
J. LI ET AL.
calculated surface area according to the BET theory was
719 m2=g for sample S4-3-0.1, with a pore volume of
1.12 cm3=g. For S1-3-0.1, the BET surface area was
824 m2=g with a pore volume of 1.05 cm3=g. The average
mesopore diameter for samples S4-3-0.1 and S1-3-0.1,
according to the Barrett-Joyner-Halenda (BJH) model,
was 4.3 nm and 3.5 nm, respectively (Figs. 3b and d). These
results indicate that the FMS indeed have a large surface
area and porous structure.
We carried out TGA and IR analyses on sample S4-3-0.1
to confirm the existence of SA molecules. The sample was
heated to 900 C in air at a heating rate of 10 C=min. The
weight loss curve (Fig. 4a) showed a 1% weight loss at about
100 C, which was likely caused by the evaporation of
residual water in the sample. There was about another 4%
weight loss from around 200 C to 400 C, which was attributed to the removal of surfactant residue (37), even though
no CH groups were detectable in the IR spectra (Fig. 4b).
FIG. 3. Nitrogen sorption isotherm of S4-3-0.1 and S1-3-0.1 FMS. (a)
Nitrogen sorption isotherm curve. The BET surface area obtained by
the single point adsorption method is 719 m2=g. (b) The pore size distribution curve based on the BJH theory. The average mesopore diameter
is 4.29 nm. (c) Nitrogen sorption isotherm curve. The BET surface area
obtained by the single point adsorption method is 824 m2=g. (d) The pore
size distribution curve based on the BJH theory. The average mesopore
diameter is 3.5 nm.
were fused together (Fig. 2c). Typically, the complexation of
silicon species with TEA resulting in silatranes requires
reaction times of several hours. The agglomeration
observed is probably due to the fast hydrolysis and condensation of TEOS (38). When the mixture had been allowed to
react for 3 h, silica particles became separated with a particle
size of 230 nm (Fig. 2a). The particle size of FMS increases
to about 400 nm for a reaction time of 24 h (Fig. 2d).
The effect of the amount of APTES-SA was also investigated. The mesoporous structure of FMS did not change
significantly when the amount of APTES-SA was small
(Figs. 2a, b, d), but the particle size decreased to about
100 nm when only one fifth of the normal amount of
APTES-SA was used (Figure 2e). The synthesized product,
however, no longer displayed the mesoporous structure or
spherical morphology when the ratio of APTES-SA:TEOS
was higher than 1:2.5 (Fig. 2f). Large pieces of irregular
particles resulted instead of mesoporous spheres. It was
further found that adding APTES, a hydrophilic organosilane, in the co-condensation reaction caused the particle
sizes of silica to increase (23).
To prove the existence of mesoporosity, the material was
precipitated with ethanol from the reaction solution,
washed with ethanol=HCl, and dried for a N2 sorption
measurement (Fig. 3). Figures 3a and 3c are the typical
isothermal curves for samples S4-3-0.1 and S1-3-0.1. The
FIG. 4. (a) Thermal gravimetric analysis curve of SA-mesoporous silica
(S4-3-0.1). red curve: TGA data, blue curve: heating profile. (b) IR
spectrum of SA-mesoporous silica (S4-3-0.1) from 400 cm1 to 4000
cm1; enlarged regions in (c) are indicated by shading. (c) IR spectrum
of the enlarged region from 1400 cm1 to 1800 cm1. (d) IR spectrum
of the enlarged region from 700 cm1 to 1500 cm1.
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MESOPOROUS SILICA FOR RADIOACTIVE MATERIAL EXTRACTION
The 9% weight loss after 400 C was attributed mainly to the
organic parts of SA and APTES. Assuming that the reactions were complete, the theoretical weight loss calculated
from the molar SA-APTES:TEOS ratio of 1:10 was
20%. These results suggested that about half of the
SA-APTES molecules were immobilized on the silica surface. IR spectra (Figs.4 b, c, and d) of sample S4-3-0.1 also
confirmed the incorporation of SA-APTES molecules. The
vibration seen at around 1050 cm1 in Fig. 4c could be
indexed as the stretch of C-O in SA. The IR bands at
around 1650 cm1 and 1550 cm1 (Fig. 4d) could be indexed
as C=O stretching and N-H bending vibrations of an amide
(Fig. 1b).
SA incorporated in the FMS produces fluorescence and
has an excellent metal coordination capability that can be
used to track FMS in the environment (41) and to remediate
radionuclide metal ions in wastewater and contaminated
groundwater. The fluorescence and UV-Vis absorption spectra were recorded using a diluted FMS suspension after the
reactant had been removed reactant by repeated centrifugation filtration and washing. The incorporated SA in
the FMS showed fluorescence at around 430 nm when excited
at 310 nm (Fig. 5a). The SA density in the sample is 9 wt.%
(functional group=silica) estimated from the fluorescent spectra. The filtrate after washing displayed very little fluorescence
(data not shown), indicating that the fluorescent group was
stable and covalently linked to the mesoporous structure.
SA is a well-known ligand with a strong affinity towards
metal ions (18). With its large surface area and pore structure, the SA-FMS showed excellent selectivity and a good
capacity to extract radionuclide ions from contaminated
solutions. Using U(VI) as an example, the equilibrium
U(VI) adsorption (Fig.
5b) follows the Langmuir adsorption model
1
Cis
¼
1
Ci max Kil
1
Ciw
þ
1
Ci max
where Cis is the
amount of metal ion U(VI) adsorbed at equilibrium per
gram of adsorbent, FMS. Ciw is the equilibrium concentration of aqueous U (VI), Cimax is the maximum number
of surface sites per gram of FMS, and Kil, which is commonly referred to as the Langmuir constant, is the equilibrium constant of the sorption reaction here. The loading
capacity Cimax that was determined from the isotherm was
3333.3 mg U(VI)=g FMS, Kil was 37.5 mL=mg, and at low
concentration range, a distribution coefficient, Kd (Kil),
was as high as 105 mL=g. This demonstrated the strong
affinity of the SA-FMS towards U(VI) in the synthetic
ground water, that has the same chemical composition as
the groundwater at the US Department of Energy’s Hanford site with a pH 8.2 and an ionic strength of 6.3 mM,
under typical groundwater U(VI) concentrations. The Kd
value is four orders of magnitude higher than the U(VI)
affinity toward most sediments in natural environments
(47–49). This result implied that the here synthesized
FMS is an efficient remediation material to capture
1511
FIG. 5. Mesoporous silica (S4-3-0.1) fluorescent spectra and isothermal
adsorption curve of uranyl ion U(VI). (a) Absorption and fluorescent
spectra of mesoporous silica. Red curve: the absorption of the filtrate,
black curve: the absorption of the silica powder, blue curve: the emission
of the filtrate, teal curve: the emission of the silica powder. (b) Isothermal
adsorption curve of uranyl ion U(VI). The curve was measured under a
condition relevant to Hanford groundwater chemical composition. The
distribution coefficient value at low concentration, Kd, was as high as
105 under typical groundwater U(VI) concentration conditions.
radionuclides, such as U(VI), from contaminated groundwater and sediments at U(VI)-contaminated sites.
The saturation loading capacity is determined by the
reactions of the uranyl U(VI) ions with other ionic species
in the simulated ground water according to the reaction
below at the specific pH 8.3 (50).
>SOH þ UO22 þ þH2 CO3 ¼> SOUO2 HCO3 þ 2Hþ
where >SOH denotes the adsorption site and
SOUO2HCO3 is adsorbed U(VI). The loading is limited
by the HCO32- concentration and is much lower than heavy
metals such as Hg.
CONCLUSION
In summary, we have synthesized a fluorescent FMS
material with a narrow particle size distribution and
ordered pore structure using a one-pot, co-condensation
reaction. The synthesized FMS may be used as a reagent
for simultaneous fluorescent tracking and radionuclide
1512
J. LI ET AL.
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remediation. The material has a Kd value of 105 (mL=g)
toward U(VI) adsorption, which is four orders of magnitude
higher than the U(VI) adsorption to most sediments in natural environments. This opens the way for a new approach
using large-surface-area mesoporous materials with desired
functional molecules for synergistic environmental monitoring and remediation in contaminated sediments.
ACKNOWLEDGEMENTS
The authors thank Dr. B. Schwenzer for helpful suggestions. This research is supported by the U.S. Department
of Energy (DOE), Office of Basic Energy Sciences, Division
of Materials Sciences and Engineering under Award
KC020105-FWP12152. The research is also supported by
the DOE, Office of Biological and Environmental Research
through a joint EPA-NSF-DOE research program. Pacific
Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle
under Contract DE-AC05-76RL01830. Juan Li thanks
the China Scholarship Council for partial financial
support. Jun Liu also thanks Wayne Cosby at PNNL for
his help in the preparation of this manuscript.
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