Gold nanoparticles-supported histamine-grafted monolithic capillaries as efficient microreactors for flow-through reduction of nitro-containing compounds

Article abstract of DOI:10.1039/c7ta00410a

A histamine functionalized monolith was synthesized within a micro-sized channel as a permeable support for the immobilization of 5, 20 and 100 nm-sized gold nanoparticles and the resulting nanostructured hybrid monoliths were applied as microreactors for the catalytic reduction of nitro-derivatives. The whole synthetic pathway of the composite materials relies on (i) UV-induced polymerization of N-acryloxysuccinimide and ethylene glycol dimethacrylate in toluene, (ii) surface grafting of histamine through nucleophilic substitution of hydroxysuccinimide leaving groups, and (iii) specific adsorption of citrate-stabilized colloidal gold nanoparticles. The achievement of the successive synthetic steps was ascertained by using a combination of experimental techniques providing information about the chemical composition (FTIR, Raman, and EDX) and porosity and surface-dispersion (SEM) of gold nanoparticles. Of particular interest, it is shown that surface-grafted histamine units exhibit strong affinity towards gold nanoparticles and allow homogeneous and dense dispersion of 5 and 20 nm sized nanoparticles. Consequently, the gold nanoparticle size-dependence of the catalytic activity (conversion of nitro and di-nitro aromatic compounds into the corresponding amino and di-amino-derivatives) was demonstrated, highlighting the utmost importance of controlling the dispersion of nano-catalysts on the support surface, while histamine protonation was also evidenced as a parameter of paramount importance regarding nanogold surface density and thus resulting catalytic activity. Histamine protonation notably allows the generation of electrostatic interactions between citrate-coated gold nanoparticles and thus-formed positive charges at the monolith surface.

Full text of DOI:10.1039/c7ta00410a

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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Gold Nanoparticle-Supported Histamine-Grafted Monolithic
Capillaries as Efficient Microreactors for Flow through Reduction
of Nitro-Containing Compounds†
Received 00th January 20xx,
Accepted 00th January 20xx
Y. Liu,a M. Guerrouache,a S. I. Kebe,a B. Carbonniera* & B. Le Droumagueta, *
DOI: 10.1039/x0xx00000x
Histamine functionalized monolith was synthesized within micro-sized channel as a permeable support for 5, 20 and 100
nm-sized gold nanoparticles immobilization and the resulting nanostructured hybrid monoliths were applied as
microreactors for the catalytic reduction of nitro-derivatives. The whole synthetic path of the composite materials relies
on (i) UV-induced polymerization of N-acryloxysuccinimide and ethylene glycol dimethacrylate in toluene, (ii) surface
grafting of histamine through nucleophilic substitution of hydroxysuccinimide leaving groups, and (iii) specific adsorption
of citrate-stabilized colloidal gold nanoparticles. The achievement of the successive synthetic steps was ascertained by
using a combination of experimental techniques providing information about the chemical composition (FTIR, Raman,
EDX), porosity and surface-dispersion of gold nanoparticles (SEM). Of particular interest, it is shown that surface-grafted
histamine units exhibit strong affinity towards gold nanoparticles and allow homogeneous and dense dispersion of 5 and
20 nm sized nanoparticles. Consequently, gold nanoparticles size-dependence of the catalytic activity (conversion of nitro
and di-nitro aromatic compounds into the corresponding amino and di-amino-derivatives) was demonstrated, highlighting
the utmost importance of controlling dispersion of nano-catalysts on support surface, while histamine protonation was
also evidenced as a parameter of paramount importance regarding nanogold surface density and thus resulting catalytic
activity. Histamine protonation notably allows the generation of electrostatic interaction between citrate-coated gold
www.rsc.org/
nanoparticles
and
thus-formed
positive
charges
at
the
monolith
surface.
controlling the size of the channel-like porous structure of
monoliths. Moreover, these porous materials are further
particularly attractive because of their high chemical stability
over a broad range of pH3 (3 to 10) together with the
combination of high permeability and fast mass transport
between the monolithic support and the surrounding fluid.4
Svec research group pioneered the use of such polymer-based
monoliths, because of their intrinsic properties (vide supra), as
Introduction
Nowadays, the development of numerous methods and
strategies to design performant microreactors is crucial
regarding the constant drive towards miniaturization and high
throughput processes, especially in the field of chemistry,
biochemistry, biology and nanotechnologies. In this context,
polymer-based monolithic reactors have shown a tremendous
interest in the past few years as they allow for the preparation
of very promising porous materials for flow through
applications. Indeed, monoliths can be prepared within
microsized channels of fused silica capillaries or chips through
simple polymerization pathway starting from a large variety of
commercially available functional meth(acrylic) monomers
allowing for the fine tuning of their chemical surface
stationary
phases
for
chromatographic
and
electrochromatographic applications.5 Since then, the
technology of monolithic stationary phases has expanded to
an unlimited plethora of solutes, regardless of their inherent
nature, i.e. hydrophobic, hydrophilic, ionic, chiral low
6-8
molecular weight molecules3,
,
peptides or proteins.9
Comprehensive and well-documented studies of chemical
engineering and separation potentiality of such functionalized
polymer monoliths can be found in different reviews in the
field, for instance.10, 11
characteristics.1, Of paramount importance, the judicious
2
choice of the chemical nature and amount of porogen(s), the
polymerization time along with the crosslinker content in the
However, the major drawback of monoliths relies in their
comparatively low surface area relative to silica-based
monolithic counterparts that may induce limited separation
efficiencies. The surface immobilization of transition or noble
metal nanoparticles on monoliths surface has been proposed
as an efficient strategy to circumvent this issue. In this respect,
gold nanoparticles have particularly attracted much interest
from scientists as their synthesis and thiol-mediated surface
polymerization feed play
a key role and enable finely
E-mail: carbonnier@icmpe.cnrs.fr, ledroumaguet@icmpe.cnrs.fr; Fax: +33 (0)149 78 12
08; Tel: +33 (0)149 78 11 77
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functionalization are well-described in the literature, as the grafting of suitable ligands at the pore surface of such in-
reported in this review from Daniel and Astruc.12 Additionally, capillary monoliths so as to address this aggregation issue
such noble metal nanoparticles are chemically inert towards encountered with the in situ strategy.
rather harsh conditions; this may notably ensure their stability Of particular interest, the immobilization of noble or transition
in time if one considers routine experiments. The group of metal nanoparticles, such as gold,19, 20 platinum21 or copper,22
Svec notably reported in 2010 on the preparation of monoliths at the pore surface of such polymer-based monolithic
with surface-immobilized gold nanoparticles.13,
14
Thiol- capillaries has allowed for the preparation of innovative flow
modified monolithic supports were prepared from a generic through catalytic microsized reactors during the last years. To
poly(glycidyl methacrylate-co-ethylene glycol dimethacrylate) extend the progress towards the development of more and
monolith through reaction of pendant epoxide moieties with more efficient catalytic microsystems, we describe the
cysteamine. Affinity of gold towards molecules and unprecedented anchoring of an imidazole containing organic
biomolecules with thiol-functional groups was exploited to the ligand, namely histamine, at the surface of polymer-based in-
successful separation of peptides and proteins as well as for capillary porous material as an efficient chelating group for the
the capture of cystein containing peptides. The same strategy robust immobilization of gold nanoparticles on monolith
16, 19
was implemented for the development of solid phase surface. Similarly to the more traditional groups, i.e. -NH2
14, 23
,
microextraction method for thiol-containing drugs such as -SH13,
and –CN 24, histamine provides full, dense and
captopril in plasma.15 Some research groups also clearly homogeneous dispersion of gold nanoparticles which is a key
demonstrated that the amine functionality allows a full, parameter for taking advantage of the catalytic properties of
homogenous and dense covalent attachment of gold nanogold. The as obtained nanostructured hybrid monoliths
nanoparticles as compared to the monolith modified with thiol were applied to the reduction of mono- and di-nitro containing
groups for which the coverage of gold nanoparticles is quite aromatic compounds in the presence of NaBH4 as reducing
sparse.16 Cystamine could also be grafted at the surface of agent.
generic GMA-based monoliths, reduced to deliver thiol
moieties that were used to immobilize GNPs in such a way that
Experimental
these later serve as intermediate ligands for the consequent
grafting of hydrophobic thiols, namely mercaptoundecanoic
acid and 1-octanethiol. The resulting hybrid monolithic
capillaries were successfully used for the separation of diverse
proteins in reverse phase-ion exchange mixed mode.17 More
recently, our group discussed a strategy based on UV-driven
click reaction to spatially control the deposition of gold
nanoparticles on monolith interface.18 (N-acryloxysuccinimide-
co-ethylene glycol dimethacrylate) monolith was first reacted
with propargylamine to provide surface-grafted alkyne groups
and further with cysteamine through radical-mediated thiol-
yne addition. Photochemical initiation affords spatial control
over the reaction site and further chelating properties of the
monolith interface. Site-specific immobilization of gold
nanoparticles was possible upon simple dynamic loading with
gold colloidal solution. The effect of the strategy employed, i.e.
ex situ vs. in situ, to immobilize GNPs onto amino
functionalized monoliths was also carefully investigated
regarding the homogeneity and density of GNPs coverage over
the monolith pore surface and its influence on catalytic activity
towards reduction of p-nitrophenol into corresponding p-
aminophenol.19 However, flow through catalytic activity of
such –NH2 functionalized gold nanoparticle-decorated
monoliths was mainly evaluated for supported catalysts
prepared via the in situ strategy involving the chelation of
Materials and Methods
N-acryloxysuccinimide (NAS), ethylene glycol dimethacrylate
(EDMA), 2,4-dinitroaniline (99%) and 2,6-dinitroaniline (97%)
were obtained from Acros Organics (Geel, Belgium). 3-
(trimethoxysilyl)propyl methacrylate, 2,5-dinitrophenol (80%),
toluene (extra dry), sodium hydroxide (NaOH), hydrochloric
acid (HCl) were purchased from Fluka (Isle-d'Abeau, France).
HPLC-grade acetonitrile (ACN), ethanol (EtOH, extra dry),
histamine (> 97%), p-nitrophenol (99%), 3,5-dinitroaniline
(97%), colloidal solutions of gold nanoparticles (GNPs) (mean
average diameter: 5 nm, 20 nm and 100 nm, stabilized
suspensions in citrate buffer) and sodium borohydride (NaBH4)
were obtained from Sigma (Isle-d’Abeau, France). 2,2′-
azobisisobutyronitrile (AIBN, 98%), purchased from Sigma, was
recrystallized from methanol prior to use and stored at ‒20 °C.
All other reagents were used without further purification.
Fused silica capillaries with a UV-transparent external coating
(75 μm id × 325 μm od) were a kind gift from InnovaQuartz
(Phoenix, AZ, USA).
Instrumentation
Functionalization, conditioning steps of the monolithic
columns and catalysis were achieved with an HPLC pump
(Shimadzu
LC-10ATVP,
Champs-sur-Marne,
France).
-
Spectrolinker XL-1500 UV crosslinker (Spectronics, Westburry,
NY, USA) equipped with six lamps (6 × 15 W, 365 nm) was used
to photo-initiate the polymerization. UV-Vis spectra were
recorded on a Cary 60 UV-Vis Spectrophotometer from Agilent
Technologies. Infrared spectra were recorded using a Bruker
Tensor 27 DTGS spectrometer in attenuated total reflection
(ATR) mode between 4000 and 450 cm‒1 with an average of 32
consecutive scans and a resolution of 4 cm‒1. Morphology of
AuCl4 as metal-precursor salt, and its subsequent reduction.
Indeed, for monolithic catalysts on which were immobilized
citrate-stabilized gold nanoparticles some aggregation trends
were noticed resulting in clogging of the capillary during the
gold nanoparticles deposition step and leading to backpressure
increase limiting the range of applicable flow rate for the
reduction of nitroaromatic compounds, hence the motivation
for this work. Herein, we have focused our investigations on
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namely toluene (700 µL). Just after the initiator AIBN (1% in
mass with respect to the total amount of monomers) was
added, the mixture was then irradiated at an overall intensity
of 8 J.cm–2 for 2 h. After the photopolymerization, the
porogenic solvent, i.e. toluene, was removed from the P(NAS-
co-EDMA) monolith by Soxhlet extraction overnight using
dichloromethane as extracting solvent; then the white powder
was dried under vacuum. Then, to 500 µL of a 1 M histamine
solution (111 mg histamine solubilized in 1 mL EtOH) were
added to 100 mg of the as-obtained porous P(NAS-co-EDMA).
The resulting dispersion was stirred at room temperature for
12 h. The dispersion was then centrifuged for 10 min at 12 000
rpm. The supernatant was discarded and the pellet was re-
suspended in EtOH. The operation was repeated at least twice
to purify the monolith that was finally dried under vacuum.
Histamine functionalization was confirmed by FTIR
spectroscopy by comparing FTIR monoliths spectra before and
after histamine functionalization.
the monolith was evaluated using scanning electron
microscopy (SEM) with the help of a MERLIN SEM apparatus
(Carl Zeiss, Germany), equipped with Inlens, SE2 and RBSD
detectors using a low accelerating tension (from 3 to 10 kV)
with a diaphragm aperture of 30 mm. Monolithic capillary
columns were cut at different places and small pieces were
deposited on a SEM support with a silver containing solution.
The samples were dried under reduced pressure and coated
with a thin layer of platinum (3 nm) with the help of a
Cressington 208 HR sputter-coater (Elektronen Optik Service,
Dortmund, Germany). Energy dispersive X-ray spectrometry
(EDX) analyses were realized with a AZtec EDS Advanced
system (Oxford Instruments society) equipped with a 50 mm2
SDD X-Max detector ensuring a good energy resolution (127 eV
for the manganese Kα ray) using a Inlens detector. Prior to
analysis, the samples were coated with a thin layer of
palladium (3 nm) with the help of a Cressington 208 HR
sputter-coater.
Bulk
monolith
purifications,
after
functionalization with histamine, were realized on a Sigma
3 15 centrifuge from Sigma Bioblock, operating at 12 000 rpm
and rt. for 10 min. Raman spectra were recorded in situ on an
XPlora One apparatus from Horiba Jobin Yvon equipped with a
laser emitting at 638 nm.
Functionalization of in-capillary poly(NAS-co-EDMA) monolith.
Chemical modification of the poly(NAS-co-EDMA) surface was
performed under dynamic conditions at room temperature.
Poly(NAS-co-EDMA)-filled capillaries were washed with EtOH
for 40 min, the monolithic columns were then allowed to react
with EtOH solutions of histamine (1 M) for 1 h. Thereafter the
monolithic columns were washed with EtOH for 15 min in
order to remove the unreacted histamine.
Synthetic Procedures
Photopolymerization of poly(NAS-co-EDMA) monoliths in
capillary. Prior to perform the in situ synthesis of the polymer
matrix under UV irradiation, the inner wall of the 100 μm
fused silica capillaries was silanized applying the following
procedure: (i) fused-silica capillaries were treated with 1 M
NaOH for 1 h at room temperature and subsequently heated
for 2 h at a temperature of 100 °C. Capillaries were flushed
with 0.1M HCl for 15 min, rinsed with deionized water for 15
min and then with acetone for 15 min. Thereafter, capillaries
were purged with dry nitrogen gas for 2 h at a temperature of
120 °C. 3-(trimethoxysilyl)propyl methacrylate (30% v/v)
solution in toluene was allowed to react overnight with inner
silanols at room temperature. At last, the capillaries were
rinsed with toluene for 15 min and dried under a stream of
nitrogen for 1 h. (ii) The reactive monomer NAS (200 mg) was
copolymerized with the crosslinker EDMA (110 μL) in the
presence of toluene (700 μL) and AIBN (1% w/w, with respect
to the total amount of monomers) as a porogenic solvent and
as an initiator, respectively. The mixture was sonicated at
room temperature to obtain a homogeneous solution; (iii)
thus, the capillary was filled with the reactive mixture, sealed
with rubber septa at both ends and irradiated at an overall
intensity of 8 J.cm‒2 for 800 s; (iv) After the photo-
polymerization process was completed, the capillary was
connected to an HPLC pump for washing with ACN (1 h at a
flow velocity of 2 μL.min–1) to remove the porogen and other
soluble compounds that did not react.
Immobilization of gold nanoparticles on the monolith surface. The
monolith was then rinsed with HCl to protonate the grafted
imidazole moieties. The adsorption of GNPs was performed
1
upon dynamic loading (2 µL.min− ) of colloidal solutions
containing 5, 20 or 100 nm GNPs and it was considered
complete when the liquid coming out from the capillary
column appeared in its original color. The specific role of the
imidazole surface functionality in the GNPs immobilization was
initially ascertained by simple visual observation of the
monolithic capillary column. The color of the histamine-
functionalized monolith became dark purple upon adsorption
of GNPs and the color remained upon thorough rinsing with
water. The capillary grafted GNPs were then used for
heterogeneous organometallic catalysis, namely for the
reduction of nitro aromatic compounds.
Reduction of nitro- and dinitroaromatic compounds by
organometallic heterogeneous catalysis by in-capillary GNPs
functionalized histamine-grafted monoliths. A freshly prepared
solution containing 200 µL of p-nitrophenol (5 mg in 10 mL
EtOH), 200 µL of NaBH4 (114 mg in 10 mL EtOH) in 4 mL EtOH
was injected in the 20 µL loop of an HPLC pump system, the
solution coming out from the in-capillary GNPs functionalized
histamine-grafted monoliths was collected and analyzed by UV
spectrophotometry. The same protocol was repeated for the
supported nanogold-mediated catalytic reduction of 2,4-
dinitroaniline, 3,5-dinitroaniline, 2,5-dinitroaniline and 2,5-
dinitrophenol.
Bulk photopolymerization of poly(NAS-co-EDMA) monoliths and
subsequent functionalization with histamine selector. Organic
polymer monolith was synthesized as follows. NAS (200 mg)
and EDMA (110 µL) were dissolved in the porogenic solvent,
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at the monolith interface via Fourrier Transform Infrared (FTIR)
Results & Discussion
spectroscopy.
In capillary monoliths preparation and characterization
Functionalization of the NAS-based monolithic columns with
histamine
A
straightforward two-step process was necessary for
preparing a histamine-grafted porous monolith within a fused
The immobilization of GNPs on monolithic capillary has been in
the past achieved on monoliths on which were tethered either
silica capillary (inner diameter = 75 µm), as shown on Scheme
1
. After activation of the inner wall of the capillary in the
presence of 3-(trimethoxysilyl)propyl methacrylate so as to
tightly anchor the monolith into the capillary (Fig. S1 ESI), a
18
14, 16, 23
amine,16, nitrile24 or thiol functionalities.13,
On the
other hand, histamine, likely due to its imidazole ring, has
been shown to interact very tightly with metal ions, but, to the
best of our knowledge, some evidence of metallic
nanoparticles adsorption to this chemical graft has never been
clearly evidenced, even if some studies hypothesized it.25
Imidazole can indeed generate two types of molecular
interactions: π-π interactions due to its aromatic ring and
electrostatic interactions due to amino groups, providing that
the pH is suitably chosen.26
,
reactive monomer, i.e. N-acryloxysuccinimide (NAS), was
copolymerized with a cross-linking comonomer, namely
ethylene glycol dimethacrylate (EDMA) in the presence of a
radical initiator (2,2’-azobisisobutyronitrile, 1% w/w, with
respect to the total amount of monomers) and a porogenic
solvent (toluene). Upon filling the pre-activated capillary with
this polymerization mixture, sealing of the capillary extremities
with rubber septa and subsequent photo-initiated
copolymerization within a UV oven, an in-capillary P(NAS-co-
EDMA) monolith was obtained with nucleophilic-sensitive N-
hydroxysuccinimide activated ester functionalities. The
presence of well-defined interconnected pores in the
micrometer-size range was demonstrated by the white optical
appearance of the capillary, suggesting the occurrence of flow-
through channels with dimensions in the same order of
magnitude than that of visible light wavelength, i.e. in the
micrometer range, as already observed in previous studies.3, 8,
In our strategy, histamine was selected as a ligand of choice as
it should allow facile adsorption of citrate-stabilized preformed
GNPs through electrostatic interaction between positively
charged ammonium moieties of histamine and negatively
charge carboxylate groups of citrate stabilizing agent coating
GNPs. In consequence, flushing a 1 M histamine solution in
EtOH through the P(NAS-co-EDMA) capillary was realized and
allowed
quantitative
consumption
of
the
N-
hydroxysuccinimide activated ester functions of the monolith,
thus allowing for the formation of covalent amide bond with
the terminal aliphatic amine moiety of histamine.
9, 18, 19, 22
This observation was also corroborated by scanning
electron microscopy (SEM) of the porous material that
displayed a homogeneous monolithic morphology constituted
This was ascertained by FTIR spectroscopy of the as-obtained
bulk monolith. Indeed, vanishing of the NAS characteristic
peaks at 1810 and 1780 cm‒1 was observed together with the
appearing of new signals assigned to the aromatic imidazole
of interconnected ∼1-2 µm-sized globules (Fig. 1) all along the
capillary length with interconnected pores in the few
micrometers range that allow for sufficient permeability and
thus fast mass transport. The monolith was also prepared in
bulk so as to further confirm the covalent grafting of histamine
1
ring (1645, 1565, 1450 and 1380 cm− ) and to –NH stretching
band of the amine (3210 cm‒1) functions, as shown on Fig. 2
,
thus demonstrating the success of the functionalization step.
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Scheme 1. Synthetic pathway followed for preparing GNPs surface-immobilized porous polymeric monoliths via free radical photo-initiated copolymerization, histamine grafting
and gold nanoparticles adsorption. The scheme is not to scale as the spheres corresponding to gold nanoparticles represent size in the 5-100 nm (depending on the size of GNPs
adsorbed at the monolith pore surface) that is much smaller than the size of monolith globules.
loading of a gold colloidal suspension through the capillary. In
order to study the influence of the acidic rinsing step and thus
of the chemical nature of the polymeric interface regarding the
adsorption of GNPs, a commercial 20 nm sized nanogold
colloid was flushed through histamine functionalized in-
capillary monolith preliminary rinsed at pH = 6.5, 3 or 1.
This successfully allowed for the adsorption of gold
nanoparticles at the pore surface within 12 h, as immediately
observed to the naked eye (Fig. S2, ESI) with the appearing of
Fig. 1. Scanning Electron Micrographs of a cross-section of the NAS-based monolithic
a homogeneous dark purple coloration all along the capillary
for pH 3 and 1 while only a faint pink color was observed with
the capillary rinsed with pH 6.5 aqueous solution. To the best
of our knowledge, this is actually the first time that chelating
properties of histamine selector towards stabilized GNPs is
clearly demonstrated. This GNPs adsorption was also
confirmed by SEM imaging and EDX spectroscopy (Fig. 4 and
Table 1, entry 2 & 3). In both cases (pH = 3 or 1), a dense and
homogeneous 20 nm GNPs coverage over histamine-grafted
P(NAS-co-EDMA) monolith was immediately noticed by SEM
capillary.
It is worth mentioning that a dramatic decrease of the
1
carbonyl peak was also noticed at 1730 cm− . This can be easily
explained by the nucleophilic substitution of surface NHS
activated esters with primary amine of histamine that release
N-hydroxysuccinimide. Compared with pristine monolith, the
histamine-functionalized one presents much less carbonyl
moieties, thus explaining the dramatic decrease of the
carbonyl peak.
(
Fig. 4). More interestingly, the monolith rinsed with pH 1 HCl
aqueous solution showed a denser gold nanoparticle coverage
than the monolith rinsed with pH HCl solution.
Quantification of gold by EDX analysis in both hybrid monoliths
confirmed this findings (Table 1 entry 2 & 3).Indeed, a much
This finding was also corroborated by in situ Raman
spectroscopy by comparing spectra of the pristine NAS-based
monolithic capillary obtained before and after grafting
reaction. It clearly demonstrated the dramatic decrease of the
3
1
peaks arising from NAS moiety after the reaction at 1815 cm−
,
1
(activated ester stretching), 1790 cm− (imide symmetric
higher gold content was observed when the capillary was
subjected to acidic washings at pH 1 (17.04 ± 4.03 wt. %) than
at pH 3 (9.69 ± 1.08 wt. %), prior to GNP immobilization. These
results are in the same order of magnitude than those already
reported in the literature for the immobilization of gold
nanoparticles over polymeric monolith, i.e. in the 15-60 wt. %
range.16, 23
1
stretching) and 1730 cm− (imide asymmetric stretching),
18, 27, 28
respectively.8,
Finally, another peak corresponding to
the –N-H amine stretching of imidazole ring also appeared at
1
3125 cm− after functionalization with histamine. It is worth
mentioning that the porous morphology of the resulting
histamine functionalized in-capillary monolith remained
unchanged after this functionalization step (data not shown).
In addition, EDX semi-quantitative analyses of the in-capillary
monolith before and after histamine grafting confirmed FTIR
and in situ Raman results. As observed on the Fig. 3, a
significant 3-fold increase in the nitrogen content (% w/w as
compared to the total amount of C, N and O elements) of the
resulting monolith was evidenced, likely due to histamine
grafting onto the P(NAS-co-EDMA) pore surface.
Prior to GNPs adsorption at the pore surface of the capillary,
the P(NAS-co-EDMA) monolith was also subjected to a rinsing
step with an acidic aqueous solution at pH = 1 or 3 in order to
protonate the available nitrogen lone pair of the aromatic
imidazole ring of histamine whose pKa was evaluated at 5.8 in
previous studies.29 This notably allowed to study the influence
of the chemical nature of the interface over gold nanoparticle
immobilization.
Fig. 2. FT-IR spectra of the P(NAS-co-EDMA) bulk pristine monolith before (black curve)
and after (grey curve) functionalization with histamine.
Immobilization of gold nanoparticles on histamine-functionalized
monolithic capillaries
The immobilization of GNPs on the histamine-functionalized
polymeric porous monolith was achieved upon dynamic
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Fig. 3. EDX spectra of in-capillary P(NAS-co-EDMA) monolith before (a) and after (b)
functionalization step with histamine.
Fig. 4. SEM images of a cross-section of the 20 nm GNP-based hybrid monoliths realized upon rinsing at pH = 1 (a, c) or pH = 3 (b, d), prior to GNPs adsorption. The
white spots represent gold nanoparticles.
Table 1. Semi-quantitative determination of wt. % content of the different adsorbed gold nanoparticles monoliths as determined by EDX analysis.
Sample
%wt Ca
68.45 ± 7.52
62.90 ± 4.28
74.31 ± 2.09
81.64 ± 1.77
47.44 ± 13.36
%wt Oa
%wt Aua
3.66 ± 2.61
17.04 ± 4.03
9.69 ± 1.08
3.98 ± 0.77
42.61 ± 15.06
monolith@5nm GNPs pH 1
monolith@20nm GNPs pH 1
monolith@20nm GNPs pH 3
monolith@20nm GNPs pH 6.5
monolith@100nm GNPs pH 1
27.89 ± 9.03
20.10 ± 0.48
15.99 ± 3.09
14.38 ± 2.52
9.96 ± 2.47
a Mean average C, O and Au content (wt. %) were determined on 3 distinct cross-sections of the hybrid monolithic capillaries.
Accordingly to previous results, the gold content of the hybrid characteristic signals from gold with Au 4f7/2 and Au 4f5/2 peaks
capillary stemming from the monolith rinsed with pH 6.5 at 83.5 and 87.4 keV, respectively.
aqueous solution was found to be as low as 3.98 wt. %.
This can be easily explained by electrostatic interactions that
Both techniques clearly pointed out the crucial role of the pH take place between the positively charged monolith interface
solution during the rinsing step prior to GNPs adsorption and and negatively charged citrate-stabilized GNPs, thus
thus the importance of histamine protonation over GNPs highlighting the crucial role of both citrate and ammonium
adsorption. This can be also verified on the EDX spectra in Fig. groups regarding the immobilization of GNPs, as already
S4, ESI in which a clear difference is observed in the gold reported in a previous study.19 In addition, SEM investigation
content upon immobilization of 20 nm sized gold nanoparticles demonstrated that this rinsing step was not destructive
after either rinsing at pH 1 or 3. XPS spectra of the bulk citrate- towards the monolith structure, methacrylate-based
stabilized 20 nm-sized GNP immobilized histamine- monoliths being inert in a broad range of pHs. Consequently,
functionalized hybrid microcolumns were also realized (Fig. S6, the rinsing step of the histamine-functionalized monolith was
ESI). Only that arising from the monolith preliminarily rinsed further realized at pH = 1 in order to improve metallic
with a pH 1 aqueous solution showed clearly visible nanoparticles immobilization on the functional porous
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material. In the same manner, the influence of GNPs diameter variation on the measurement was observed. In addition, the
over monolith coverage was further investigated. In this 100 nm GNP adsorption step was much more difficult at the
purpose, 5 nm GNPs colloids were flushed through the pore surface of such monoliths as only nearly 3 cm of the
histamine functionalized P(NAS-co-EDMA) capillary preliminary capillary was decorated with GNPs after 48h. This
rinsed at pH 1. It only allowed a weak coverage of the pore demonstrated the crucial role of the GNP size over the
surface of the monolith as shown on Fig. 5a, even if the coverage density on the porous polymeric surface, and thus on
immobilization along the entire capillary was achieved within 4 the gold specific surface area increasing. Indeed, as observed
h. Unfortunately, a much lower gold content, i.e. 3.66 ± 2.61 in Fig.5b, 20 nm GNPs seem more suitable than 5 and 100 nm
wt. %, was found by EDX semi-quantitative analysis. As GNPs as derivatives in the presence of NaBH4. They allowed for
observed by SEM (Fig. 5c), a disperse coverage of the obtaining dense GNP distribution while avoiding any
histamine grafted monolith was noticed in the case of 100 nm aggregation phenomena.
GNPs along with some aggregation patterns (Fig. S3, ESI). This Flow through supported catalysis within hybrid microcolumns
was also confirmed by EDX analysis that pointed out a great Immobilized GNPs are prone to act as organometallic catalysts
variation of the gold content of the hybrid monolith, notably in various reactions, among which the reduction of
depending on the area on which the analysis was realized, i.e. nitroaromatic compounds to the corresponding amino. In this
in GNPs aggregation areas or in less densely distributed areas work, we thus decided to focus our attention on this particular
(Fig. S5, ESI). Nonetheless, an average gold content of 42.61 ± supported GNP-mediated catalytic reaction.
15.06 wt. % could be calculated, even though a great standard
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Fig. 5. SEM images (RBSD detector, left) and corresponding EDX spectra (right) of (a) 5 nm GNPs-, (b) 20 nm GNPs- and (c) 100 nm GNPs-adsorbed histamine-grafted monolith
interfaces. Insets show higher magnification micrographs of the 5-nm and 20-nm GNPs immobilized hybrid materials realized with Inlens detector. The white spots on SEM images
represent
GNPs
To this purpose, a solution of p-nitrophenol and NaBH4 in EtOH nitro compound into the corresponding amino compound. We
was injected in a 7 cm length 5 nm GNPs immobilized hybrid thus decided to collect the eluted solution at the outlet of the
monolithic capillaries at increasing flowrates, namely 2 and 5 capillary and to analyze it by UV-visible spectroscopy. It was
μL.min‒1. An excess of NaBH4 was used in our experimental immediately observed that the characteristic band of p-
procedure. Accordingly, the reduction of such nitrocompounds nitrophenolate at 400 nm decreased when the solution was
over supported GNPs do not depend on the concentration of passed through GNPs immobilized hybrid capillary. The
this reducing agent.30,
31
The eluting yellowish solution conversion of each catalytic reaction was calculated by
constituted of p-nitrophenol and NaBH4 injected at the inlet of comparing the absorbance of the solution before and after
the GNPs-supported monolithic capillary was found totally nanometal-catalyzed reaction in the hybrid microcolumn at a
faded at the outlet, thus suggesting a total conversion of the wavelength corresponding to the πꢀπ*transition peak of the
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reactant, i.e. (di)nitroaromatic compound. It is worth
mentioning that for each reaction, a negative blank in which
the nitro-compound was injected in a histamine-grafted
monolith was used as a negative control. In this way, it was
also possible to prove that the presence of supported GNPs is
necessary for the catalytic reaction to occur and that no
dilution affects the absorbance of the nitro-based reagent or
amino-based product. More interestingly, it was noticed that
decreasing the flowrate of the HPLC pump allowed for a better
conversion of the nitroaromatic compound, as demonstrated
Fig. 6. UV spectra of p-nitrophenol solution supplemented with NaBH4 after flushing
through histamine grafted P(NAS-co-EDMA) capillary ((a) and (b), black curve), through
5 nm GNPs immobilized capillary at a 5 μL.min‒1 ((a), red curve) or at a 2 μL.min‒1
flowrate ((a), green curve) or through 20 nm GNPs immobilized capillary at a 5 μL.min‒1
by the dramatic decrease of the characteristic π
p-nitrophenolate ion at 400 nm (Fig. 6.a) in the case of 5 nm
immobilized GNPs and a strong increase of the π π* band of
ꢀπ* band of
flowrate ((b), red curve). Capillaries were all 7 cm length and the pump flowrate was
ꢀ
1
.
varied from 2 to 5 µL.min−
p-aminophenolate at 300 nm. Conversions of 53% and 64%
were thus obtained with immobilized 5 nm-sized GNPs hybrid
1
microcolumns for flowrates of 0.5 and 0.2 µL.min− ,
respectively. This can be easily explained by longer contacts
between nanogold and reagents at lower flowrates, thus
enabling higher nitro reduction yields.
Further, the GNP size effect was investigated by comparing
two different hybrid capillary columns, namely those prepared
after immobilization of 5 nm or 20 nm GNPs on the histamine
grafted monolith. As expected, it immediately demonstrated
the better efficacy of the 20 nm GNPs immobilized column
over the 5 nm one. Indeed, when the same flowrate was used
for both columns, i.e. 5 μL.min‒1, the conversion of the p-
nitrophenol into the corresponding amino-containing
compound was virtually quantitative when using hybrid
monolithic column generated with 20 nm GNPs (Fig. 6.b). As
aforementioned, in the case of immobilized 5 nm GNPs, the
reduction yield was found to be only 53% by comparing the
absorbance of the negative control with those of the GNPs-
mediated reaction. However, when comparing the conversion
obtained from hybrid monolithic columns generated with 5
Fig. 7. UV spectra of (a) 2,4-dinitroaniline, (b) 3,5-dinitroaniline, (c) 2,6-dinitroaniline
and (d) 2,5-dinitrophenol upon flushing through histamine-grafted monolith (red curve)
or 20 nm GNPs-immobilized monolith (black curve). Capillaries were all 7 cm length and
1
.
and 20 nm GNPs as a function of the gold content in the the pump flowrate was fixed at 5 µL.min−
column, it is obvious that 5 nm GNPs immobilized hybrid
For this purpose, a series of 4 dinitroaromatic compound was
selected: 2,4-dinitro-, 3,5-dinitro-, 2,6-dinitroaniline and 2,5
dinitrophenol. As shown in Fig. 7, the hybrid capillary column
exhibited good catalytic activity with excellent yields toward
this series of dinitroaromatic compounds regardless of the
nature and position of the substituents on the aromatic ring.
Indeed, after recovery of each solution at the capillary outlet
and ex situ analysis by UV-visible spectroscopy, a quantitative
reduction was observed as demonstrated by the vanishing of
the characteristic band ascribed to the π → π* electronic
transition of these dinitro- compounds, as indicated by a
dashed arrow.
capillaries show better conversions (53% conversion for 3.66
wt. % in the case of 5 nm GNPs vs. 99% conversion for 17.04
wt. % in the case of 20 nm GNPs). Thus the surface to volume
ratio is more favorable in the case of 5 nm GNPs and allows a
higher surface contact with reactants, leading to higher
conversions. Besides dense coverage of the histamine-grafted
monolith interface with 100 nm GNPs, a high back pressure
and even clogging of the capillary in some cases did not allow
for using such microsystems in the hydride-mediated
reduction of nitro-containing aromatic compounds, explaining
the lack of results with this system.
To demonstrate the recyclable character of the hybrid capillary
columns, the same flow through catalytic reaction was
repeated 5 times in a row with the hybrid in-capillary monolith
obtained after immobilization of 20 nm GNPs. conversions up
to 96 % after 5 cycles demonstrated the possibility to use such
hybrid systems for repeated processes.
Conclusions
In this contribution, we have proposed the surface-grafting of
a naturally-occurring amine, i.e. histamine, onto N-hydroxy-
succinimide-functionalized monolith with the aim to produce
nanostructured monoliths acting as microreactors for the
catalytic reduction of nitroaromatic derivatives. As
demonstrated in this work, two crucial parameters have to be
Furthermore, we investigated the possibility to reduce di-nitro
aromatic compounds with such prepared 20 nm GNPs-
immobilized hybrid monoliths.
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Journal Name
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seriously taken into consideration for the rational design of
such catalytically active hybrid materials, i.e. the average
diameter of the nanometal colloid and the nature of the
porous material interface. In this particular case, best results in
terms of permeability and nanometal content were obtained
with 20 nm GNPs and highly positively charged interface. For
the 20 nm sized particles, homogeneous and dense surface
dispersion of the metallic colloid was obtained. The gold
content of the resulting hybrid monolith was estimated by EDX
analysis and found to be as high as 17 wt. %. The proposed
approach is proved to be a promising method for complete
conversion of reactants within a few minutes within short
hybrid monolithic capillaries. Such hybrid capillary columns
could thus be easily envisionned for facile screening of
catalytic reactions, regardless of the nature of the transition or
noble metal used. Moreover, it can be extended to other types
of chemical reaction as well as other nanometals. Besides, the
adsorption of such GNPs onto the porous monolith surface
may enable to increase the specific surface area of such
monoliths and thus the interactions with solutes for capillary
(electro)chromatography for instance. Finally, another
prospect of such hybrid capillaries relies on the reduction and
consecutive quantification of trinitrotoluene (TNT), the
systems being thus used for the qualitative as well as
quantitative analysis of this explosive.
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Acknowledgements
The authors wish to acknowledge the Université Paris-Est and
CNRS for financially supporting this research. S. I. K. is grateful
to the Ministry of Higher Education and Scientific Research of
the Government of Senegal for financial funding during his
PhD. We would also like to thank Rémy Pirès for his assistance
in the SEM/EDX microscopy measurements.
2012, 8, 378.
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