Functionalized Nanoporous Silica for the
Removal of Heavy Metals from Biological
Systems: Adsorption and Application
Wassana Yantasee,*,†Ryan D. Rutledge,‡Wilaiwan Chouyyok,‡Vichaya Sukwarotwat,‡
Galya Orr,‡Cynthia L. Warner,‡Marvin G. Warner,‡Glen E. Fryxell,‡Robert J. Wiacek,‡
Charles Timchalk,‡and R. Shane Addleman*,‡,§
Department of Biomedical Engineering, OHSU School of Medicine, Portland, Oregon 97239, and Pacific Northwest
National Laboratory (PNNL), Richland, Washington 99352
ABSTRACT Surface-functionalized nanoporous silica, often referred to as self-assembled monolayers on mesoporous supports
(SAMMS), has previously demonstrated the ability to serve as very effective heavy metal sorbents in a range of aquatic and
environmental systems, suggesting that they may be advantageously utilized for biomedical applications such as chelation therapy.
of removing selected heavy metals from biological solutions (i.e., blood, urine, etc.) Consequentially, thiol-functionalized SAMMS
was further analyzed to assess the material’s performance under a number of different biologically relevant conditions (i.e., variable
pH and ionic strength) to gauge any potentially negative effects resulting from interaction with the sorbent, such as cellular toxicity
or the removal of essential minerals. Additionally, cellular uptake studies demonstrated no cell membrane permeation by the silica-
based materials generally highlighting their ability to remain cellularly inert and thus nontoxic. The results show that organic ligand
functionalized nanoporous silica could be a valuable material for a range of detoxification therapies and potentially other biomedical
KEYWORDS: nanoporous • mesoporous • SAMMS • thiol • iminodiacetic acid • biocompatibility • heavy-metal chelation •
sorbent • ion exchange • detoxification
pore morphology and installation of a wide range of surface
chemistries. The ability to design and build such functional
nanoporous materials from elements (silicon and oxygen)
that are not intrinsically toxic is attractive for biomedical
applications. Nanoporous silicas are of particular interest
capacity for the adsorption and desorption of molecular and
ionic species (1). Furthermore, the well-ordered porosity
enables rapid and controlled release or capture of small
molecules (2). The porosity and structure can even be
arranged to immobilize biomolecules so that they retain
much of their activity (3-8). The flexible surface chemistry
and biocompatibility have enabled mesoporous silica ma-
terials to be used to facilitate such biologically sensitive
doped with dyes has been shown to be valuable for monitor-
anoporous silica provides a material support struc-
ture that is hydrothermally stable while retaining
enough malleability to allow precise control of the
ing of various biological processes and even imaging of
internal cellular processes (5, 9, 12-20). Controlled drug
release from mesoporous materials has been extensively
explored (21-33). Modified mesoporous materials have
recently been reported for the directed and controlled
delivery of anticancer drugs such as titanocene complexes
(9, 34-37). In an effort to expand the biomedical applicabil-
ity of these types of materials, the use of functionalized
the treatment of heavy metal exposure.
Industrial activities have driven the wide-spread refine-
ment and utilization of heavy metals with the unfortunate
concurrent releases of these toxic materials into the environ-
ment. Heavy metal remediation, detoxification, and assay
are of significant interest because exposure to metals like
cadmium (Cd), lead (Pb), mercury (Hg), and arsenic (As) are
known to cause a range of diseases that are detrimental to
been shown to enable the rapid diagnosis of heavy metal
also be beneficial, and advanced materials may offer a
metals from the body involves organic chelators such as
ethylenediaminetetraacetic acid (EDTA) and 2,3-dimercap-
* Tel.: 503-418-9306. Fax: 503-418-9311. E-mail: email@example.com.
Received for review July 15, 2010 and accepted September 27, 2010
†OHSU School of Medicine.
‡Pacific Northwest National Laboratory.
§Tel.: 509-371-6824. Fax: 509-375-2186. E-mail: firstname.lastname@example.org.
2010 American Chemical Society
Published on Web 10/12/2010
VOL. 2 • NO. 10 • 2749–2758 • 2010
tosuccinic acid (DMSA). However, these methods require
and damage the renal system as the metal chelates are
cleared from the body. In addition, the chelation agents are
known to extract essential metals (i.e. iron, magnesium,
calcium, and zinc) from the body, resulting in mineral
deficiencies, which can produce a number of detrimental
deaths have recently been reported that were attributed to
cardiac arrest resulting from hypocalcemia due to EDTA
providing adsorption from the gastrointestinal tract would
and facilitate their fecal elimination. Further, oral adminis-
tration eliminates the need for medical professionals to be
involved in the treatment while enabling convenient and
administration of Prussian blue (insoluble iron ferrocyanide
from the human body (52). A solid-phase sorbent capable
of effectively capturing toxic species directly from relevant
biological fluids, while ignoring essential metals, would
clearly be therapeutically advantageous.
Self-assembled monolayers on mesoporous silica (SAMMS
is a registered trademark) has been shown to be a very
effective heavy-metal sorbent in aqueous matrixes, outper-
carbon under most conditions (53-55). SAMMS has the
advantage of nanoporous silica (high surface area and open
porosity) combined with a high-density, covalently bound
(1, 56, 57). Diverse surface functionality can be installed to
adjust the sorbent selectivity for the capture of target materi-
als (58-61). Heavy metal chelation surface chemistries
analogous to EDTA and DMSA have been installed and
Anionic heavy metals such as chromate and arsenate can
also be captured by the installation of cationic transition-
metal complexes (64-66). SAMMS success as a sorbent in
aquatic media is well-documented and suggests a potential
utility in biological systems. However, the shift in applicabil-
ity from aqueous matrixes encountered in environmental
samples to biological fluids is not a trivial matter because of
the complex composition of biological fluids (e.g., high
protein content, potential cellular interactions, etc.).
Applications needing toxin capture from biological fluids
are not limited to heavy metal chelation. A number of
situations can be envisioned in which high-efficacy solid-
phase sorbents called upon to aid in the removal of toxic
species from biological fluids would be therapeutically ben-
eficial. For example, an increase in the use of gadolinium
has been linked to a potentially fatal skin disease, nephro-
genic systemic fibrosis, in some patients, and the removal
of excess or free Gd from the system should be advantagous
(67, 68). Radioisotopes coupled with cancer binding agents
are currently being used as a treatment for a range of
cancers, and the removal of free radioisotopes not bound
to cancerous tissue would significantly reduce systemic side
effects or enable increased dosages for equivalent side
effects (69-72). Platinum compounds have demonstrated
a remarkable effectiveness at fighting a number of cancers
but are quite toxic, which limits their dosages and utilization
(73-75). Analogous to radioisotopes, the removal of free
platinum compounds that did not bind to the cancer could
would enable dialysis-style treatments without placing any
burden upon the kidneys (67). Clearly, for many applica-
tions, the removal of excess, unbound, or unneeded materi-
als could reduce side effects, toxicity, and lingering symp-
toms (70). Heavy metal removal, as well as other potential
applications, is dependent upon the presence of a sorbent
material that has the affinity and selectivity for effective
toxin capture under very challenging solution conditions
(i.e., high salt, high protein, low concentration of toxin, etc.)
during the timeframe that the sorbent is exposed to the
This work explores the sorption efficacy and biocompat-
ibility of SAMMS-based materials in biological matrixes. As
was previously mentioned, SAMMS can be made to bind
transition metals, lanthanide, or a range of radionuclides,
but herein we focus upon toxic heavy metal applications.
We have investigated the material performance for select
heavy metals as a function of the biological matrix (blood/
plasma, urine, synthetic gastrointestinal fluids, etc.) and
degradation and cellular uptake. General issues with the
utilization of functionalized nanoporous silica for potential
therapeutic and diagnostic applications are also examined.
Sorbent Materials. The synthesis and characterization of the
self-assembled monolayers on mesoporous silica (SAMMS)
materials have been described elsewhere, including thiol (SH)-
SAMMS, acetamide phosphonic acid (AcPhos)-SAMMS, glyci-
nylurea (Gly-Ur)-SAMMS, and IDAA-SAMMS, which is based
analogously to the chelating ligand EDTA (1, 56, 57, 62, 76).
The large-pore MCM-41 was synthesized based on the method
of Sayari and co-workers (77). Specific surface areas and pore
sizes were determined using an Autosorb-6B surface area
analyzer (Quantachrome Corp., Boynton Beach, FL) using the
Brunauer-Emmett-Teller and Barrett-Joyner-Halenda rou-
tines, with pore size values determined from the desorption
data. The ligand density was determined by analysis of the
organic content of the materials post-functionalization using a
NETZCH STA 409 C/CD thermogravimetric analyzer. Figure 1
presents the chemical structures of the SAMMS materials
utilized in this study. Some of the basic material properties of
these sorbents have been summarized in Table 1. Commercial
resin values are not shown because the flexible polymer struc-
ture precludes accurate measurement of their surface area. The
SH resin used in this study was GT-73, a thiol-functionalized
styrene-divinylbenzene resin sorbent manufactured by Rohm
and Haas Co. (Philadelphia, PA). The EDTA resin used in this
benzene sorbent manufactured by Bio-Rad. The activated
carbon was Darco KB-B (from Sigma-Aldrich, Milwaukee, WI).
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Test Matrixes. Batch metal sorption experiments were per-
formed in a number of different solutions. Human urine and
blood were used as received, though the blood (Golden West
Biologicals, Inc., Temecula, CA) contained 0.1 M sodium citrate
as an anticoagulant. Rat urine was diluted 4 times prior to use.
The synthetic gastric fluid (SGF) and synthetic intestinal fluid
U.S. Pharmacopeia for drug dissolution studies in stomach and
intestine, respectively (78, 79). The SGF (pH 1.11) contained
0.03 M NaCl, 0.085 M HCl, and 0.32% (w/v) pepsin. The SIF
contained 0.05 M KH2PO4; its pH was adjusted to 6.8 with 0.2
M NaOH. Pancreatin, a protein component, was omitted from
the SIF formula because it was shown to clog filters and render
analysis impractical. A modified Krebs-Henseleit buffer solu-
tion (pH 6.80) consisted of 118.0 mM NaCl, 4.7 mM KCl, 1.2
mM MgSO4, 1.2 mM KH2PO4, 11.0 mM D-glucose, 2.5 mM
CaCl2·2H2O, and 25.0 mM NaHCO3. All reagents were pur-
chased from Sigma-Aldrich and were of the highest purity
Batch Metal Adsorption. Batch metal adsorption experi-
ments were used to determine the affinity of a sorbent for a
target metal species. The affinity was then quantitated via
calculation of a distribution coefficient (Kd, mL/g; eq 1),
where C0and Cfare the initial and final concentrations of the
target species as determined by inductively coupled plasma
mass spectrometry (ICP-MS), V is the matrix volume, and M is
the mass of the sorbent.
Each experiment was performed as previously described (a
detailed account can be found in the Supporting Information)
(80). A liquid sample was spiked using metal ion standards to
obtain a known concentration of each target metal (As, Cd, Hg,
and Pb). A small amount of the sorbent material suspended in
deionized water was then added to the samples to obtain a
desired solid-to-liquid ratio (S/L with units of g/L throughout).
The sample was allowed to mix for 2 h at 160 rpm on an orbital
shaker, after which it was centrifuged and the supernatant
collected for analysis. The metal concentrations were deter-
mined via an Agilent 7500 inductively coupled plasma mass
spectrometer after construction of calibration curves using the
four metals. Each experiment was performed in triplicate.
Material Stability. Along with the Kdmeasurements, Si was
measured via ICP-MS in the solutions before and after batch
per gram of material was reported for each solution matrix as
the average value of three replicates.
In Vitro Caco-2 Cell Uptake. Caco-2 cells were seeded for
21 days at 37 °C and 5% CO2in a transwell polycarbonate
of this apparatus is shown in the Supporting Information). SH-
SAMMS was prebound with 1.0 mg of Cd, 1.0 mg of Hg, 1.0
mg of Pb, and 0.6 mg of As per gram of SH-SAMMS prior to cell
exposure (see the Supporting Information). The solid was
suspended in a transport buffer (pH 7.4) consisting of 1.98 g/L
glucose, 10% (v/v) 10× Hank’s balanced salt solution with
calcium (Ca) and magnesium (Mg), and 0.01 M 4-(2-hydroxy-
ethyl)-1-piperazineethanesulfonic acid (HEPES) buffer at a S/L
ratio of 10 g/L. A 0.25 mL aliquot of this suspension was added
to the apical side of the cell monolayer, and 1.0 mL of the
transport buffer sans sorbent was added to the basolateral side.
After 2 h, the solution from the basolateral side was collected
and diluted 10-fold in 2% HNO3for ICP-MS analysis of the four
metals (As, Cd, Hg, and Pb) and Si. Each experiment was
performed in triplicate with two controls (without metal-bound
Cell Monolayer Integrity. The integrity of the Caco-2 cell
after contact with the metal-bound SH-SAMMS was assessed by
measuring the transendothelial electrical resistance (TEER) of
After 2 h of incubation time, the solution in the basolateral side
the cell membrane was measured with the electrical cell sensor
system (World Precision Instruments, Sarasota, FL). Two con-
trols were performed using the same Caco-2 cell systems but
without the addition of metal-bound SH-SAMMS.
Cell Uptake Study Using Fluorescent Dye-Tagged SH-
SAMMS. SH-SAMMS was tagged with a fluorescent dye (Alexa
Fluor 488, Invitrogen, Carlsbad, CA) according to our coupling
procedure (see the Supporting Information). For this study, the
Caco-2 cells were grown in-house (see the Supporting Informa-
tion for the culture procedure), transferred to 1.5 mL of trans-
port buffer, and exposed to fluorescent dye-tagged SAMMS at
for 3 h, the cells were imaged while alternating between
correlated differential interference contrast (DIC) and fluores-
cence modes on a wide-field Axiovert system from Ziess. The
cells were then incubated with Trypan Blue (Sigma) for 15 min
at room temperature in order to quench the fluorescence,
washed with the transport buffer, and imaged again to deter-
mine the particle internalization.
RESULTS AND DISCUSSION
Comparative Sorbent Performance in Biologi-
cal Matrixes. In an effort to identify an optimum sorbent
material for heavy metals in biological systems, various
silica-based sorbents were tested for their ability to capture
As, Cd, Hg, and Pb ions in both blood- and urine-based
matrices. Blood and urine are both biologically relevant test
FIGURE 1. Schematic of the organic ligand monolayers utilized in
Table 1. Sorbent Properties (Values Given for
SH-SAMMS (large pore)a
aUnfunctionalized large-pore silica had a surface area of ∼1000
m2/g and a pore size of ∼7.5 nm.
surface area of ∼870 m2/g and a pore size of ∼5.0 nm.
cUnfunctionalized silica had a surface area of ∼750 m2/g and a pore
size of ∼3.2 nm.
bUnfunctionalized silica had a
www.acsami.orgVOL. 2 • NO. 10 • 2749–2758 • 2010
matrices that were used to establish a sorbent’s therapeutic
different functionalized nanoporous silica sorbents selected
for this work (shown in Figure 1 and Table 1) are known to
be effective at binding heavy metals, although the different
surface chemistries result in different binding mechanisms,
which impacts the affinity and selectivity. The absolute and
including the porosity of the silica support, surface area,
ligand density, and interaction of the material with the
sample matrix. The smaller size of the thiolpropyl ligand
(Figure 1) enables higher surface ligand densities (Table 1),
which likely contribute to its superior performance (shown
subsequently). The ability of these materials to bind and
remove heavy metal ions in both blood and urine is pre-
sented in Tables 2 and 3, respectively. The affinity of the
sorbent for a target species is represented in terms of a
distribution coefficient, Kd(mL/g), which is a mass-normal-
ized partition coefficient between the solid sorbent phase
and the liquid solution phase. Kdis an experimental value
and after metal sorption has occurred. Typically, the higher
the Kdvalues, the better the sorbent. It should be pointed
out that proteins are known to complex metal ions; thus, a
large portion of the heavy metals in biological solutions will
metal ions are known to interact with the various forms of
inorganic (e.g., carbonates and phosphates) anions present
in the biological fluids. Consequently, the metal speciation
can be very complex and will depend strongly upon the
specific analtye and solution conditions. High efficacy sor-
bents must be able to outcompete the other reactive species
in solution for the available metal ions.
The values presented in Table 2 highlight thiol-based
SAMMS as the superior sorbent with regards to heavy-metal
capture from blood. Although overall the Kdvalues remain
generally low because of the complex nature of the matrix,
SH-SAMMS binds each metal approximately 2 orders of
magnitude better than most of the other sorbents (most
notably the commercial thiol resin and activated carbon).
The urine sorption data in Table 3 are less decisive, but a
similar trend is observed with SH-SAMMS, representing the
best general sorbent for the metals tested. These results are
somewhat expected because the softer thiol ligands would
be predicted to bind the softer heavy metals better than the
harder, more oxygenated ligands installed on the other
bound metals in the urine matrix with a lower affinity than
both the SH-SAMMS and other commercial materials and
proved incapable of binding metals in blood. This is an
interesting result because one might expect closely related
and functionalized silica because of their similar surface
of factors but is principally due to the higher surface area
coupled with the highly ordered monolayer interface of the
SAMMS sorbent, allowing multiple metal-ligand interac-
tions, as well as the greater tendency of the polymeric resin
to accumulate protein fouling. The high-density monolayer
allows metal cations to interact with multiple thiol groups,
resulting in stronger binding interactions as opposed to the
randomly ordered copolymer resin, in which the metal
cations are most likely interacting with a single thiol group.
has a range of biological applications. Both commercial and
SAMMS-based EDTA-like sorbents proved capable of captur-
ing metals from the biological solution and showed similar
composed of carboxylic acids and amines. As a result, these
sorbents were better at capturing the harder transition
metals like Cd and Pb than the softer metals like As and Hg.
AcPhos- and Gly-Ur-SAMMS were designed specifically to
bind rare-earth cations and, not surprisingly, showed little
to no binding for Cd, Hg, or Pb in either solution, thus
demonstrating that these ligand systems have too low of an
affinity to be effective sorbents for “soft” heavy meals in
biological matrixes (although they may be good ligands for
the selective capture of rare-earth cations in biological
the softer heavy metals. Activated carbon has a high surface
area but possesses harder ligands (e.g., carboxylates, phe-
nols, etc.), which are less ordered, resulting in a more
random coordination with the metal ions and thus a lower-
affinity surface chemistry. Activated carbon was only effec-
tive at capturing Pb from the blood matrix but was capable
Table 2. Affinity (Kd) of As, Cd, Hg, and Pb for
Various Sorbents in Bloodb
Kd(mL/g) in blood
aPore sizes are presented in Table 1.
metal concentration of 50 µg/L (each), S/L of 1 g/L, pH 7.5.
bMeasured at an initial
Table 3. Affinity (Kd) of As, Cd, Hg, and Pb for
Various Sorbents in Urineb
Kd(mL/g) in urine
aPore sizes are presented in Table 1.
metal concentration of 50 µg/L (each), S/L of 1 g/L, pH 6.92.
bMeasured at an initial
VOL. 2 • NO. 10 • 2749–2758 • 2010 Yantasee et al.www.acsami.org
of binding Cd, Hg, and Pb in urine. Only for the capture of
Pb in urine was activated carbon an improvement over SH-
The Kdvalues in Tables 2 and 3 show that thiol-function-
alized mesoporous silica will serve as an effective heavy-
metal sorbent in biological systems. The data from Table 2
show SH-SAMMS to be a vastly preferred sorbent material
delivered treatment, the material must meet the following
criteria: it must have high affinity for the target metals
among the nontarget metals in an assortment of relevant
matrices (e.g., blood, urine, low-pH gastric fluid, near-
neutral-pH intestinal fluid, etc.), sufficiently rapid metal
binding rates, large sorption capacity (e.g., not saturated
with the nontarget metals), long-term stability so as not to
facilitate the release of captured metal ions, and the ability
the nonspecific adsorption of proteins, and it must resist
cellular uptake while not damaging the cell. These criteria
are investigated for therapeutic and diagnostic applications
in subsequent sections.
Sorbent Affinity as a Function of the Sample
of sample matrices. Matrices were chosen not only with
regard to their biological relevance but also to help gain an
understanding of metal uptake as a function of the protein
content, pH, and ionic strength. Biologically relevant solu-
tions included dilute rat urine, normal human urine, whole
and SIF, respectively). Both synthetic gastric and intestinal
fluids used in this work were prepared according to the
formula recommended by the U.S. Pharmacopeia for drug
dissolution study in mammals (78, 79). Table 4 compares
the Kdvalues obtained for the four metals and SH-SAMMS
measured in these solutions.
Previous work has shown that thiol-SAMMS has a high
affinity for Hg, Cd, and Pb in natural waters, with Kdvalues
in excess of 104and occasionally exceeding 106(81). Inspec-
tion of the data in Table 4 shows that Kdvalues are smaller
in biological matrices than in natural waters. SH-SAMMS
shows a decrease in the metal uptake for experiments
performed in urine and blood compared to natural waters.
One possible explanation for the reduction in the Kdvalues
in natural water versus biological matrices is with regard to
the available protein content. As the relative protein con-
centration of the sample matrix is increased (dilute urine <
concentrated whole urine < blood), the Kdvalues are gener-
ally observed to decrease.
The term “biofouling” is often used to describe a drop in
the performance or efficiency of a material due to the
presence of a large quantity of biomolecules. The increased
presence of protein is capable of negatively impacting the
sorbent performance in a number of ways. Physisorption
of the biomolecules to either the interior or exterior surface
of the mesoporous silica could result in significantly de-
creased ligand-metal interaction. Protein interaction with
the available thiol ligands is also conceivable because of the
large degree of disulfide linkages and other potentially
reactive sites present throughout protein structures. Preven-
tion or reduction of biofouling caused by any (or all) of these
mechanisms would lead to improved sorbent performance
in biological fluids, thus expanding their applicability.
Also of note is the increased performance of the large-
pore SH-SAMMS relative to the smaller-pore samples in the
of potential pore blockage. The data indicate the greater
uptake ability of the large-pore SAMMS possibly due to the
decreased pore blockage that arises simply from having
larger pores. Decreased pore blockage allows greater access
of the metal ion to the bulk of the silica surface area inside
the pores, resulting in increased ligand-metal binding.
These results indicate the need for a more extensive study
this increased uptake, however, the large-pore SAMMS still
experiences a large degree of protein fouling, as evidenced
by the considerable loss in uptake ability relative to experi-
ments performed in “protein-free” matrices (Supporting
Information). In order to eventually realize the optimal
potential of these materials for heavy metal detection and
detoxification therapeutics, efforts must be made to reduce
Sorbent Affinity as a Function of the pH. Any
potential oral therapeutic is going to be subjected to a range
of solution conditions on its journey through the body.
of these changing conditions is vital toward achieving a
complete understanding of the material’s therapeutic po-
tential. The solution pH is one of these conditions, ranging
Table 4. Affinity (Kd) of SH-SAMMS for As, Cd, Hg, and Pb in Various Fluidsb
Kd(mL/g) in various matrices
conductivity (mS/cm)pH available protein contentAsCd HgPb
dilute urine (rat)
whole human urine
whole human urine (large pore)
whole human blood
whole human blood (large pore)
7.9 × 104
1.1 × 104
1.0 × 104
2.0 × 103
3.3 × 103
1.7 × 102
7.7 × 105
2.8 × 104
5.6 × 103
2.3 × 104
1.2 × 103
7.6 × 103
1.5 × 106
2.7 × 105
5.6 × 103
1.9 × 103
1.1 × 103
1.3 × 103
1.9 × 103
2.0 × 104
3.5 × 103
5.0 × 103
4.2 × 103
4.2 × 103
1.7 × 104
2.7 × 104
aSilica pore sizes noted in Table 1.bMeasured at an initial metal concentration of 50 µg/L (each), S/L of 0.2 g/L except for urine, blood, and
plasma, S/L of 1 g/L.cSGF ) synthetic gastric fluid.dSIF ) synthetic intestinal fluid.
www.acsami.orgVOL. 2 • NO. 10 • 2749–2758 • 2010
from highly acidic to slightly basic (pH ∼1-8.5), depending
on the region of the body. For example, the pH range of the
biological matrices discussed in this report range from pH
1.1 in SGF to pH 7.5 in blood (Table 4) and is known to be
as high as 8.3 in various regions of the gastrointestinal tract.
The Kdvalues of As, Cd, Hg, and Pb on SH-SAMMS as a
function of the solution pH are reported in Figure 2. The
solutions were prepared by adjusting the pH of the SGF (an
initial pH of 1.11) with 0.2 M NaHCO3in order to achieve a
regions of the gastrointestinal tract (pH 1-3 in stomach, pH
in jejunum and ileum) (82). It should be noted that the SH-
SAMMS affinity for all analytes remains stable or increases
with rising pH, which correlates to the pH trend in the
gastrointestinal tract, suggesting that captured metals will
not be leached out by the changing intestinal environments.
The high binding affinity (Kd ∼ 106) observed for Hg
across the entire pH range is expected and has previously
been reported for SH-SAMMS in acidic wastewater (1). This
binding consistency indicates that the thiol surface remains
active throughout the pH range encountered in the gas-
trointestinal tract. While speciation in the biological solution
can be complex, the observed pH-dependent adsorption of
Cd and Pb ions can likely be attributed to the hydrolysis
behavior of these ions over the pH range because a similar
behavior has been observed in aquatic matrices (83-85).
The Kd value of As was high at pH 1.1 (Kd ∼ 17 000),
decreased to by a factor of 10 as the pH increased from 1.1
to 4.0, and rose again as the pH became more neutral
(5.6-8.3). As the pH increases, the emergence of negatively
charged species (i.e., H2AsO4-and HAsO42-) becomes more
prevalent, with the divalent nature of the latter species
allowing for interaction with multiple thiol ligands, thus
resulting in a significant increase in the overall affinity, as
observed with Cd and Pb. The increase in Kdat lower pH for
As is not completely understood, but the data do suggest a
high affinity between completely protonated thiols and
neutral arsenic species, H3AsO4and H3AsO3, which are the
dominant species under acid conditions (86).
Sorbent Affinity as a Function of the Ionic
Strength. Several oral drugs rely on ion-exchange resins
for capturing undesirable toxins, such as sodium poly(sty-
potassium, and cross-linked allylamine hydrochloride, an
anion-exchange resin for binding with phosphate in the
gastrointestinal tract (87). Unfortunately, however, polymer
resins have been known to suffer from swelling and shrink-
ing caused by a variation in the solution ionic strength,
threatening to retard the therapeutic properties of these
resin-based drugs. The ionic strengths experienced by a
sorbent in biological matrices tend to be greater than that
of most natural waters (seawater being the primary excep-
tion). Conductivity measurements, in general, show a 10-
as blood and urine, compared to river water and ground-
water (Table 4 and Supporting Information). In order to
evaluate the performance of SH-SAMMS as a function of the
ionic strength, heavy metal uptake was measured through-
out a range of ionic strengths, achieved by varying the
concentration of a sodium acetate buffer matrix. The data
for SH-SAMMS indicate that increasing the concentration of
the sodium acetate buffer from 0.001 to 0.1 M has very little
effect on the affinity of the sorbent for the four metals
(Supporting Information), consistent with similar results in
ionic solutions (56). Only when the acetate concentration
was increased to 1 M did the affinity for Pb and Cd decrease
by about 10-fold, while still retaining a relatively high Kd
high throughout the range of ionic strengths. Consequently,
variations in the biologically relevant ionic strengths are
Adsorption Kinetics and Capacity. Fast sorption
kinetics are typically therapeutically and diagnostically ben-
be particularly valuable because it limits the amount of time
that toxins would be bioavailable, thereby limiting the
amount absorbed into the human body. Detailed sorption
kinetics of Hg in SGF and of Cd in SIF are available in the
Supporting Information. Over 99% of Hg in SGF and Cd in
SIF were removed after 3 min. This rapid sorption rate is
due to the rigid pore structure and mesoporous size, which
allows constant exposure to a greater number of available
thiol binding sites, in contrast to the swellable polymer ion-
exchange resins such as GT-73 (54). From 2 to 24 h of
contact time, the extent of sorption remains steady, indicat-
ing that there is no significant leaching of Hg and Cd from
the laden sorbent and no significant degradation of the
materials in these two matrices.
Adsorption capacity data were obtained for Hg in SGF,
Cd in SIF, and As in both SGF and SIF (Supporting Informa-
tion). The matrices were chosen on the basis of their
affinities for binding SH-SAMMS exhibited in Table 4. The
adsorption data for each metal are very consistent with a
FIGURE 2. Kdof As, Cd, Hg, and Pb, measured on SH-SAMMS in
synthetic gastrointestinal fluids prepared by adjusting SGF with 0.2
M NaHCO3to the desired pH. Initial metal ion concentration ) 50
µg/L. S/L ) 0.2 g/L. Error bars given represent standard deviations
and, when not visible, indicate a variance of less than the data icon
VOL. 2 • NO. 10 • 2749–2758 • 2010 Yantasee et al. www.acsami.org
Langmuir adsorption model (R2> 0.99), strongly suggesting
out of the solutions at these conditions. The maximum
sorption capacities for Hg, Cd, and As, as estimated by
Langmuir isotherm models are summarized in Table 5.
While the measured capacities in Table 5 are lower than
very good because of the high affinity of SH-SAMMS for the
target metals. The reduction of capacities is likely due, in
part, to the presence of high concentrations of other ions in
the test solutions (NaCl, KCl, and KH2PO4) when compared
to less complex matrices such as distilled water or ground-
water. Furthermore, the acidity of the SGF increases the
protic competition for the binding sites (compared to near-
neutral solutions), resulting in a lower overall capacity
(1, 88). A similar trend has been observed in less complex
matrices such as acidified water (1).
The adsorption isotherm of Pb in SIF could not be
measured in the same fashion with others because of the
However, on the basis of the affinity data in Table 4, Pb is
predicted to have an absorption capacity between those of
the other metals. Table 5 also indicates that an increase in
the temperature from room temperature (24 °C) to body
temperature (37 °C) has little effect on As sorption on SH-
SAMMS, which is generally expected for covalent bonding
or monolayer chemical adsorption. Thus, most batch sorp-
tion experiments in this work were measured at room
Material Stability. Whereas material stability might
not be very important for short-term sorbent applications
discussed herein, would seek to limit material degradation.
For this reason, the extent of SAMMS stability was deter-
exposed to the SAMMS sorbent. The weight percents of Si
dissolved per total mass of SH-SAMMS after 2 h of stirring
in acidic (pH 1.1) and near-neutral (pH 6.8) fluids were
measured. Very little leaching was observed, 0.2% (pH 1.1)
and 2% (pH 6.8) Si loss by weight, showing good material
stability for the tested conditions. Because of the strong
as well as the high degree of cross-linking among the thiol
monolayers, the Si dissolved is suspected to come from the
residual (and physisorbed) poly(mercaptopropylsiloxane)
used in the SH-SAMMS synthesis rather than the monolayer
degradation (the latter would result in a decrease in the
binding capacity). The materials also possess a good shelf-
life, having demonstrated the same metal binding perfor-
lifetimes probable (67).
Cell Uptake and Cell Integrity. In order to serve as
a potential detoxification therapeutic, the sorbent materials
must not only maintain a high affinity for the target metals
but also remain relatively inert with regard to the types of
cells and biomolecules encountered in the body. An ideal
material would minimize cellular interaction in a manner
that avoids damage to the cell while preventing release of
the captured metals. Consequently, cellular uptake studies
were performed to assess the level of interaction between
cells and SAMMS material in vitro. Caco-2 cells were utilized
for these studies because they possess many of the proper-
ties of the small intestinal epithelium and have been used
previously to determine transport of chemicals across the
human intestinal epithelium (89-91).
material to maintain metal chelation was assessed. The SH-
SAMMS material that had been prebound with 1.0 mg/g
sorbent each of Cd, Pb, and Hg and 0.6 mg/g of As was
suspended in solution for 30 min. No detectable leaching of
Cd, Pb, and Hg and only a small leachate of Si and As (0.1
this period of time, indicating the ability of the SAMMS
material to retain bound metals. For cellular uptake studies,
Caco-2 cells were cultured for 21 days in a transwell poly-
carbonate membrane culture dish before use to investigate
the transport of SAMMS across the epithelial cells. Cell
membrane permeability is evaluated through the cell’s
ability to grow into a monolayer atop a permeable filter
support. Material is added to either the apical or basolateral
side of the monolayer and allowed to incubate. Mass spec-
trometry analysis of the solution from both sides then gives
This type of assay is often used to make in vivo predictions
as to the bioavailability of a potential drug based on in vitro
For this study, the preloaded SAMMS material was sus-
pended in a transport buffer, added to the apical side of the
monolayer, and allowed to incubate for 2 h. Samples for
detection were then taken from the basolateral side. The
metal concentrations detected in these samples are listed
in Table 6 and represent a measure of the ability of the
material to permeate and transport across the cell mono-
layer. The data indicate no difference in the concentrations
of the four metals on the basolateral side between the test
and control groups (with no metal-bound SAMMS material
added), signifying a lack of cell permeability. This ability to
resist cellular uptake is likely due to the relatively large
5 µm, and the mean particle size is 22 µm, compared to a
Table 5. Maximum Capacity of Metal Uptake by
SH-SAMMS As Predicted by the Langmuir Sorption
metal max capacity (mg/g)matrixb
SIF, 37 °C
aAll fits with regression better that 0.99.
contained 0.03 M NaCl, 0.085 M HCl, and 0.32% (w/v) pepsin; SIF
contained 0.05 M KH2PO4. The pH was adjusted to 6.8 with 0.2 M
bSGF (pH 1.11)
www.acsami.orgVOL. 2 • NO. 10 • 2749–2758 • 2010
cell diameter of approximately 10 µm) because previous
studies have indicated that materials with much smaller
dimensions (∼50 nm in diameter) tend to be susceptible to
uptake. TEER measurements (Table 6) were taken to gauge
the integrity of the monolayer postincubation with the
SAMMS material. No difference between cells that were
suggesting that the metal-bound SAMMS are not damaging
A series of DIC and fluorescence images, taken through
the z-axis of the cells after exposure to fluorescent dye-
tagged SH-SAMMS, confirm this lack of uptake. The fluores-
cently tagged material was incubated with the cells for 3 h,
followed by fluorescence quenching by Trypan Blue (Figure
3). Although larger particles (>5 µm) can be observed to be
stuck to the cell surface, none of these particles seem to
µm) appear to enter the cell cytoplasm. Also of note, no
change in the morphology of the cells was observed in the
cells (not shown).
Chelating of Essential Minerals. One of the draw-
backs of EDTA chelation that is commonly employed for
detoxification therapy is that it facilitates urinary excretion
of essential minerals, especially Ca (by 2-fold) and Zn (by
by SH-SAMMS was examined using similar concentrations
encountered in vivo. We found that SH-SAMMS did not
significantly remove 100 mg/L of Ca, 30 mg/L of Mg, 0.5
mg/L of FeIII, and 0.5 mg/L of Mo in both the low-pH SGF
Kreb’s buffer, was used in this study to better solubilize the
minerals of interest). This observation is in line with the
hard-soft acid-base (HSAB) theory that predicts the soft
thiol ligand would have a very low affinity for hard metals
such as Ca, Mg, and Fe (92). A total of 0.5 mg/L of Zn and
Cu was also investigated; however, the metals could largely
be collected by 0.45 µm filters (even in the absence of
Table 6. Uptake Study of Metal-Bound SH-SAMMS with a Caco-2 Cell Monolayer
TEER (Ω cm2)
metal-bound SAMMS (µg)a
test group (ng)b
% metal transport
1.4 ( 0.03
1.2 ( 0.01
2.1 ( 0.15
2.5 ( 0.00
0.3 ( 0.11
0.4 ( 0.12
2.5 ( 0.00
1.3 ( 0.43
0.9 ( 0.00
2.5 ( 0.00
0.5 ( 0.16
0.1 ( 0.03
798 ( 16
792 ( 19
aEstimated from 0.0025 g of SH-SAMMS bound with 0.6 mg of As and 1.0 mg of Cd, Hg, and Pb per g of SH-SAMMS.bMetals in the
basolateral side of Caco-2 cells after 2 h of contact with the material in footnote a, in triplicate.cMetals in the basolateral side of Caco-2 cells
after 2 h of contact with the filtrate of metal-bound SAMMS suspension (after solid removal), in duplicate.
FIGURE 3. DIC image (1) showing the presence of two large dye-tagged SAMMS particles (about 10 and 25 µm, indicated by the stars) on the
cell surface. As images are taken deeper in the cells (2-4), the particles disappear. The correlated fluorescence image (1a and 2a) shows dim
or no fluorescence signals from the dye-tagged particles as a result of fluorescence quenching by Trypan Blue. Together, these observations
indicate that the large particles stay at the cell surface. The fluorescence images that are taken deeper in the cells (3a and 4a) show the
presence of a bright spot, which indicates that a small particle (∼1-2 µm) was internalized into the cytoplasm, where it was protected from
the quenching by Trypan Blue.
VOL. 2 • NO. 10 • 2749–2758 • 2010 Yantasee et al. www.acsami.org
sorbent), making it difficult to assess their uptake on SH-
will be measured in a future in vivo study. However, we
predict that both Zn and Cu, which are borderline metals
according to the HSAB principle, would likely be captured
by SH-SAMMS but to a much lower degree than “soft” heavy
a much lower affinity for Zn than for Hg support this
A number of commercial and mesoporous silica-based
sorbent materials with various surface chemistries were
analyzed in vitro to assess their viability as oral therapeutics
SAMMS proved to be the most effective sorbent materials
for heavy-metal sorption from biological matrices such as
blood, urine, and synthetic gastric and intestinal fluids. The
performance of the thiol-functionalized material remained
relatively unperturbed throughout relevant pH and ionic
strength ranges and was shown to only minimally degrade
over an extended period of time in both neutral and acidic
pH values. Additionally, cellular uptake studies demon-
strated no cell membrane permeation by the silica-based
materials, generally highlighting the materials cellular bio-
compatibility. Finally we found that SH-SAMMS did not
remove essential elements such as Ca, Mg, and Fe from
bodily fluids, thus offering an improvement over the more
traditional EDTA chelation methods whose nondiscrimina-
tory metal-binding properties have been known to cause
death. Taken in concert, these results suggest that improve-
ments in heavy metal chelation and detoxification therapies
may be achieved through high-performance orally admin-
istered high-efficacy sorbents, such as those demonstrated
with SH-SAMMS for the softer heavy metals. A preclinical
evaluation to assess the in vivo efficacy of the SH-SAMMS
gauge the efficacy of these materials. Additionally, we
acknowledge that while the SH-SAMMS sorbent did prove
the most capable material for removing the majority of the
available heavy metals in biological solutions, there remains
an opportunity for additional performance improvement.
Further investigations aimed at understanding and minimiz-
ing the deleterious interactions (e.g., protein interference)
in biological fluids as well as improving material efficacy
through improved surface chemistry and material structure
are currently underway. Different surface chemistries are
also being explored for other theranostic applications. This
focused heavy metal detoxification study is an initial proof
of concept demonstration. The design and development of
advanced sorbent materials with high efficacy in biological
fluids should enable a broad range of beneficial therapeutic
methods and diagnostic tools.
Acknowledgment. The work was supported by the Na-
tional Institute of Environmental Health Sciences (grant R21
ES015620) and the National Institute of Allergy and Infec-
tious Disease (Grants R01-AI074064 and R01-AI080502).
The research was performed, in part, at the Environmental
Molecular Sciences Laboratory, a DOE national scientific
user facility located at PNNL. The authors thank View Koon-
George A Porter, Dr. Worapon Kiatkittipong, Dr. Joongjai
Panpranot, and Dr. Karla Thrall for their contributions.
Supporting Information Available: Additional experi-
mental detail, metal contact data in “protein-free” matrixes,
metal uptake, and SH-SAMMS kinetic studies and capacity
measurements in biological fluids. This material is available
free of charge via the Internet at http://pubs.acs.org.
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