Catalytic Materials Based on Surface Coating with Poly(ethyleneimine)-Stabilized Gold Nanoparticles

Article abstract of DOI:10.1002/cctc.201700776

Gold nanoparticles (AuNPs) can be obtained from HAuCl₄ by using poly(ethyleneimine) (PEI) as both reductant and stabilizing agent. However, the known affinity of PEI for different materials has not been exploited to coat them and turn their surface catalytic. We demonstrate that the irradiation of a solution of HAuCl₄ and branched PEI 1800 (bPEI2K) with microwave (MW) yields PEI-stabilized AuNPs (MW-PEI@AuNPs) with an average size of 7.6 nm that are catalytically active in the reduction with NaBH₄ of different nitroarenes functionalized with a variety of functional groups. Moreover, the as-prepared MW-PE@-AuNPs show affinity for different materials such as polystyrene (standard spectrophotometry disposal cuvettes), polypropylene (Falcon-type tubes), and silica (Silica gel 60), turning their surface catalytic without any additional synthetic step. This feature was exploited to transform standard tubing (Tygon, poly(ether ether ketone), and stainless steel) into flow reactors by simple passage of a solution of MW-PEI@AuNPs. This straightforward functionalization is especially appealing in the case of the stainless-steel tubing, one of the materials more widely used in HPLC, which is of interest for flow nanocatalysis.

Full text of DOI:10.1002/cctc.201700776

Accepted Article
Title: Catalytic Materials Based on Surface Coating by PEI-Stabilized
Gold Nanoparticles
Authors: Mariano Ortega-Muñoz, Victor Blanco, Fernando Hernandez-
Mateo, F. Javier Lopez-Jaramillo, and Francisco Santoyo-
González
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10.1002/cctc.201700776
ChemCatChem
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Catalytic Materials Based on Surface Coating with PEI-Stabilized
Gold Nanoparticles
Mariano Ortega-Muñoz, Victor Blanco, Fernando Hernandez-Mateo, F. Javier Lopez-Jaramillo,* and
Francisco Santoyo-Gonzalez*[a]
Abstract: Gold nanoparticles (AuNPs) can be obtained from HAuCl4
by using polyethyleneimine (PEI) as both reductant and stabilizing
agent. However, the known affinity of PEI for different materials has
not been exploited to coat them and turn their surface into catalytic.
We demonstrate that the irradiation of a solution of HAuCl4 and
branched PEI 1800 (bPEI2K) with microwave (MW) yields PEI-
stabilized AuNPs (MW-PEI@AuNPs) with an average size of 7.6 nm
that are catalytically active in the reduction with NaBH4 of different
Despite metallic gold was historically identified as a noble
metal chemically inert toward chemisorption and discarded for
catalysis, the pioneering works by Haruta et al. changed this
vision.[8-10] Nowadays, catalytically active gold nanoparticles
(AuNPs) are prepared either by loading a gold precursor (usually
a gold salt or complex) onto a solid support followed by chemical
reduction or calcination to promote the reduction of the gold
cations into elemental gold that forms NPs, or via traditional
-
nitroarenes functionalized with
a
variety of functional groups.
colloidal chemistry by the reduction and stabilization of AuCl4
Moreover, the as-prepared MW-PE@-AuNPs show affinity for
different materials such as polystyrene (standard spectrophotometry
disposal cuvettes), polypropylene (falcon-type tubes) and silica
(Silica gel 60), turning their surface into catalytic without any
additional synthetic step. This feature is exploited to transform
standard tubings (Tygon, PEEK and stainless steel) into flow
reactors by simple passage of a solution of MW-PEI@AuNPs. This
straightforward functionalization is especially appealing in the case
of the stainless steel tubing, one of the materials more widely used
in HPLC, being of interest in the context of flow nanocatalysis.
with reductants and stabilizing agents that prevent aggregation,
such as polymers, surfactants or ligands.[11-12] Colloidal AuNPs
can be used either directly as catalysts in liquid phase or
deposited onto solid supports (i.e. colloidal deposition).[1-3]
AuNPs have been supported on materials such as TiO2, ZnO,
Al2O3, SiO2, CeO2, Fe2O3, metal-phosphonates, carbonaceous
supports and stainless steel (SS).[14-16] However, neither the
stabilizing ligands nor the support may be passive elements and
they can block the reactive surface sites or influence the
characteristic of the NPs by tuning the oxidation state of the gold
or by adsorbing the reactants or intermediates/products,
provoking the activation of the reaction or the deactivation of the
catalyst.[17-19]
Introduction
In some cases, both reduction and stabilization can be
achieved by the use of a single specific polymer such as
polyvinylpyrrolidone (PVP), polyethylene glycol (PEG),
polyamidoamine (PAMAM), poly(propyleneimine) (PPI) or
polyethylenimine (PEI), among others.[20-24] In particular, linear
PEI was the first polyelectrolyte reported with this dual role by
Sun et al. in 2004[21] but it is branched PEI (bPEI) with a
molecular weight of 25 kDa the most widely used.[21-24] It has
been demonstrated that PEI acts as an efficient reductant of
HAuCl4 at both 60-80 ºC and room temperature, the size of the
resulting NPs being dependent on the bPEI:HAuCl4 ratio.[21-24]
Additionally, PEI, as a polymeric amine with a high density of
charge, has been used to improve the adhesion of different
compounds onto surfaces as diverse as clay, polystyrene, silica,
glass, graphite and metal oxides such as SnO2, ZrO2 or TiO2.[25-
Nanocatalysis is
a
domain at the interface between
homogeneous and heterogeneous catalysis and it is based on
the use of nanomaterials and nanoparticles (NPs). The catalytic
activity of the NPs is influenced by multiple variables, among
them the number of surface atoms, especially those located at
the corners and edges, and the size of the NPs.[1-3] Hence, NPs
need to be stabilized to prevent aggregation. Alloying,
encapsulation or the use of stabilizing agents (i.e. capping
agents) such as polymers, surfactants or ligands in colloidal
chemistry are common stabilization strategies.[4-5] However,
stabilization may exert an influence on the reactivity, morphology
or surface chemistry and, in this context, the advent of
nanotechnology has facilitated not only the study of NPs at
atomic resolution but also the techniques to control the overall
shape, composition, morphology and uniform size.[6-7]
30]
However, to the best of our knowledge the affinity of PEI has
not been fully exploited to extend its role and synthesize
stabilized AuNPs with the capability of coating different
surfaces.[25-30] Recently we have reported the one-pot synthesis
of PEI coated AuNPs that combines the reductant/stabilizer
properties of PEI and the use of use of microwave irradiation to
yield DNA delivery nanocarriers.[31] In this work we explore the
ability of these AuNPs to coat surfaces and turn them into
catalytic materials.
[a] Dr. M. Ortega-Muñoz,Dr. Victor Blanco; Prof. F.J. Lopez-Jaramillo,
Prof. F. Hernandez-Mateo, Prof. F. Santoyo-Gonzalez
Department of Organic Chemistry, Biotechnology Institute, Faculty of
Sciences, University of Granada
18071 Granada (Spain)
Fax: (+34)-95824
E-mail: fjljara@ugr.es ;fsantoyo@ugr.es
Supporting Information and the ORCID identification number(s) for the author(s)
of this article can be found under http://dx.doi.org/10.1002/ctcc.xxxxxx.
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Results and Discussion
conditions for a variety of reactions.[1,33] This size dependence
has been found to be related with the fraction of low coordinated
Au atoms that decreases as the size of the NPs increases,
suggesting that the atoms on the corners and edges are the
active site.[1] Among the different approaches to reduce the size
of the AuNPs, two are straightforward: i) increasing the PEI:Au
ratio, and ii) increasing the temperature of reaction. The former
leads to a significant suppression of the primary nucleation
whereas the latter yields a higher activity of the amino groups in
reducing Au3+.[34] Since the assayed PEI:Au ratio of 18.7 yielded
a suitable NP size, close to the optimum values, we decided to
focus on the temperature and used MW irradiation. Unlike
conventional thermal techniques, MW is characterized by a
more homogeneous heating that provides rapid initial heating
and rapid consumption of the starting material which yields
smaller NPs with a narrow particle size distribution in shorter
reaction times.[35-37] Thus, based on our previous experience,
MW irradiation of a mixture of HAuCl4and bPEI2K using the
optimal 18.7 PEI(repeating units):Au ratio at 300 W and 100 ºC
for 3 min turned the solution into dark red, the colour being
stable for weeks.[31] The UV-Vis spectrum revealed a SPR peak
at 521 nm (Figure 2) and the average size of these NPs, named
MW-PEI@AuNPs, was estimated from expression (1) as 7.2
nm.
Synthesis and Characterization of MW-PEI@AuNPs
PEI was the first polyelectrolyte reported as both reducing and
stabilization agent in the synthesis of AuNPs[21-24] and we have
recently demonstrated31 that a short microwave (MW) irradiation
of a solution of PEI and HAuCl4 yields PEI-stabilized AuNPs
(PEI@AuNPs) that can be used as DNA delivery nanocarriers.
However, to the best of our knowledge the known affinity of PEI
for different materials has not been exploited to coat surfaces
with PEI@AuNPs and turn them into catalytic.25-30 We focused
on branched PEI with molecular weight of 1800 (bPEI2K) as a
key compound for the triple purpose of reducing, stabilizing and
coating. First we analyzed the influence of both the PEI
(repeating units):Au ratio and the MW irradiation. As expected
from literature,[21] our preliminary experiments at room
temperature showed that a PEI(repeating units):Au ratio of 9.3
yielded PEI@AuNPs that aggregated but that were stable when
the amount of HAuCl4 was reduced to increase the ratio to 18.7.
The synthesis was monitored by UV-Vis spectroscopy and after
30 h the surface plasmon resonance (SPR) peak was detected
while the colour of the reaction medium evolved from yellow to
dark red, supporting the formation of dispersed PEI@AuNPs.
However, as the synthesis progressed the reaction medium
darkened and the SPR peak showed a red shift from 530 nm to
548 nm, suggesting the formation of larger PEI@AuNPs (Figure
S1). Since the optical properties of AuNPs are dependent on the
size and the wavelength, the size of AuNPs was estimated with
an expected error of ~11% from the ratio between the
absorbance of the SPR peak and that at 450 nm by applying the
expression (1), that is applicable in a 4 to 100 nm range.[32]
d= e(3.0∗(Aspr/ A450)− 2.2)
(1)
A closer analysis of the spectra using expression (1)
revealed a clear trend with the size increasing as the synthesis
progressed to reach a plateau at 12 nm after 60 h of reaction in
agreement with the red shift of the SPR peak and the observed
darkening of the reaction medium (Figure 1).
Figure 2. SPR peak (left) and HRTEM image (right) from a MW-PEI@AuNP.
The characterization of MW-PEI@AuNPs by transmission
electron microscopy (TEM) demonstrated that they are
homogeneous, with an average size of 7.6±1.5 nm, being 0.20
the coefficient of variance (Figure S2), in good agreement with
the size estimated from the UV-Vis spectrum. HR-TEM images
also suggest that these NPs are nanocrystals with high
crystallinity (Figure 2 right). Considering the ratio between the
NP volume and the volume per atom in the bulk metal
(assuming the same packing) the number of atoms per NP was
estimated as 14122-14165, which is very close to the magic
number 14993 for an icosahedron or cuboctahedron with 17
shells and 2562 surface atoms.[38,39]
Fourier transform Infrared spectroscopy (FTIR) provides
additional insight into the transformations that PEI suffers during
the synthesis of the MW-PEI@AuNPs (Figure 3). Similarly to our
PEI@AuNPs reported as efficient DNA delivery nanocarriers,[31]
the most characteristic feature of the FTIR spectrum of MW-
PEI@AuNPs are the bands at 1631 cm-1 and 1557 cm-1 absent
in bPEI2K and that were tentatively assigned to the C=O
stretching and the N-H bending of amide groups respectively.
Additionally, the shape of the peak at 3280 cm-1 assigned to the
N-H stretching of amino groups changes and shifts to 3343 cm-1
whereas the signal at 1598 cm-1, corresponding to N-H bending
Figure 1. Red-shift of the SPR peak during the synthesis of PEI@AuNPs
(PEI(repeating unit):Au ratio=18.7, room temperature) and estimation of the
size of the NPs according to expression (1). [32]
It is well established that the size of the NPs is a critical
parameter in nanocatalysis and that in the particular case of
AuNPs, diameters of ~5 nm are catalytically active under mild
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Table 1. Catalytic efficiency of MW-PEI@AuNPs in the reduction of 4-
nitrophenol (4-NP) 4-nitroaniline (4-NA), 4-nitroacetanilide (4-NAc) by
NaBH4.
Nitroarene
tTON[a] (%activity)
TOF (min-1)
4-NP
4-NA
4-NAc
1.33×10-14 (18.2%)
5.35×10-15 (19.2%)
4.17×10-15 (17.8%)
4.84×10-16
5.35×10-16
3.09×10-16
[a] Total turnover number (moles of substrate per mol of surface atom)
calculated as the sum of TON obtained in each run of catalyst recycling
until the catalytic activity dropped to the value in brackets
sources of uncertainty: i) the definition of the time when
thecatalyst is considered inactivated and ii) the challenge of
defining mol of catalyst with NPs due to the importance of atoms
placed at specific positions, specially those located at the
corners and edges.[1-3,38] An additional difficulty is the fact that
soluble metal NPs are surrounded by agents/ligands that may
limit the accessibility of the reactant to the surface atoms.[38]
Bearing in mind all these limitations we have estimated the total
turnover number (tTON) defined as the sum of TON obtained in
each run of catalyst recycling until the catalytic activity dropped
below 20% and we have assumed that only surface atoms are
responsible for the catalysis and that all of them are equivalent.
Our data show that the reduction of 4-NP is faster than that of 4-
NAc and slower than that of 4-NA but the catalyst becomes
deactivated sooner (Table 1).
Figure 3. IR spectra of MW-PEI@AuNPs (red) and bPE2K (blue) depicting
representative peaks.
of amino groups shifts to 1557 cm-1 as expected from the
formation of amides. Moreover, the spectrum of MW-
PEI@AuNPs shows a signal at 2100 cm-1 that may be tentatively
41
attributed to CO linearly bonded to AuNPs.40, These findings
support the transformation of amino groups into amides and the
role of amines as reductants to convert Au3+ into Au0.[42]
Catalytic activity of MW-PEI@AuNPs
Among the different reactions that AuNPs catalyze, the
reduction of nitroarenes by NaBH4 is relevant because nitro-
aromatics are an important class of environmentally toxic
compounds widely employed in the production of explosives,
pesticides, plastics and pharmaceuticals and the resulting
amines can find application in many fields, underlying the
opportunity to carry on new studies to continuously improve the
reduction efficiency.[43,44] Additionally, the transformation of 4-
nitrophenol (4-NP) in 4-aminophenol (4-AP) is a model reaction
widely used to study the efficiency of metallic NPs.[45-47] On this
basis, the reduction of 4-NP, 4-nitroaniline (4-NA) and 4-
nitroacetanilide (4-NAc) were assayed as model reactions to test
the catalytic activity of the MW-PEI@-AuNPs. In these
experiments the decrease of the strong absorption at 400 nm,
380 nm and 319 nm characteristic of each of these starting
materials was observed. In the presence of a large excess of
NaBH4 and an adequate amount of MW-PEI@AuNP the reaction
can be described by a pseudo-first-order kinetics and analyzed
using the Langmuir-Hinshelwood model. The catalytic property
can be quantified by the apparent rate constant (kapp) obtained
from the slope of the linear correlation of ln(At/Ao) with time.
This model involves the adsorption of both the reducing agent
and the nitroarene on the surface of the NPs and it assumes that
the diffusion of the reactants to the NPs and all the adsorption-
desorption steps are fast, being the reduction of the adsorbed
nitroarene the rate-limiting step.[45-47] MW-PEI@AuNPs are
catalytically active in the reduction of 4-NP, 4-NA and 4-NAc,
with an induction time of 1-1.5 min (Figure S3). Many authors
have attributed the observation of an induction time to a slow
surface reconstruction of the NPs, usually ascribed to the
diffusion/adsorption time for 4-NP previous to the start of the
reaction.[45-47] The kinetic parameters are summarized in Table
1.
As already discussed, NPs aggregation is a major concern
that it is circumvented by alloying, encapsulation or by the use of
capping agents.[4,5] The as-prepared MW-PEI@AuNPs are
stable in solution, which has an alkaline pH, and catalytically
active for weeks. It has been suggested that the degree of
protonation of PEI has a strong influence on its conformation,
being more elongated at strong acidic conditions as
a
consequence of steady increase in the electrostatic
a
interactions that cause repulsions of the repeating units.[48] On
the basis of this hypothesis the effect of acidic pH was
evaluated. The incubation of MW-PEI@AuNPs at different
concentrations of HCl yields a red-shift and a broadening of the
SPR that was dependent on the time of incubation. Moreover,
pH values lower that 3 (i.e. 30 mM HCl) provoked the
precipitation of the MW-PEI@AuNPs (Figure S4). The
evaluation of the catalytic activity after incubation with 30 mM or
60 mM HCl for 60 min reveals that, although this short
incubation has not a significant influence on the size or the
induction time, kapp diminishes significantly (Table 2). These
results led us to conclude that MW-PEI@AuNPs are not stable
at strong acidic pH and, although the aggregation is not
instantaneous, the effect on the catalytic performance is
relatively fast.
Table 2. Influence of the incubation during 60 min with different
concentrations of HCl on the size, kapp and induction time (to) in the
reduction of 4NP to 4AP by NaBH4 catalyzed by MW-PEI@AuNPs
[HCl] mM
Estimated Size (nm)[a)
kapp
to(min)
0
7.2
6.9
7.5
0.405
0.210
0.177
1.5
30
60
1.5
It is important to recall that TON is defined as the number of
moles of substrate that a mol of catalyst can convert before
becoming inactivated and its estimation has to deal with two
1.0
[a] Estimated from the UV-Vis spectra according to expression (1).32
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To further assess the performance of the MW-PEI@NPs, the
reduction of a series of nitroarenes with structurally diverse
functional groups was evaluated. The reaction was allowed to
proceed for 1 h at room temperature and an aliquot was
quenched by addition of formic acid and analyzed by HPLC-MS.
Results are summarized in Table 3. In all cases except for 4-
nitrobenzonitrile and 4-NAc (Table 3, entries 5 and 12) more
than 95% of the starting material was transformed within 1 h. In
order to evaluate whether these substrates were less reactive
the reaction was studied as a function of time. The percentage
of 4-NAc consumed after 1, 2.5, 5.5 and 8 h of reaction was
72%, 83%, 93% and 95%, respectively, confirming the kinetic
parameters previously estimated that demonstrate the lower
reactivity of 4-NAc when compared to 4-NP and 4-NA. However,
for 4-nitrobenzonitrile the fact that the transformation was
affected neither by the reaction time nor by the addition of extra
NaBH4 at 2.5 and 5.5 h of reaction led us to conclude that the
catalyst was inactivated.
An attempt to identify the products resulting from the
reduction was carried out by searching the mass of all the
intermediates of the general mechanism of the reduction of
nitrobenzene proposed by Haber and accepted by the scientific
community.[49] Haber´s mechanism is a multistep pathway where
nitroso and hydroxylamine intermediates are formed to yield the
direct reduction to the amino group (i.e. direct route) or a
condensation to yield azoxy and azo derivatives (i.e.
condensation route). The intermediates identified among the
different species formed for the different nitroarenes assayed
are depicted in Table 3.
For 4-nitroaniline, 4-nitrobenzoic acid, 4-nitrobenzonitrile, 4-
nitrobromobenzene, 1-chloro-2,4-dinitrobenzene and 4-NAc
(Table 3, entries 2, 4-6, 10 and 12) only the amino derivatives
Table 3. Reduction of nitroarenes by NaBH4 catalyzed by MW-PEI@AuNPs and intermediates identified[a].
[c]
Entry
Substrate
Conversion[b] AOB[c] AB[c] ABH2
A[c]
Comments
1
99%
Chemoselectivity
✔
2
3
99%
✔
✔
✔
96%
Chemoselectivity
✔
4
5
6
7
95%
28%
97%
99%
Chemoselectivity
Inactivation of the catalyst
✔
✔
Identification of products failed
Chemoselectivity
8
99%
✔
✔
✔
✔
✔
✔
✔
9
99%
99%
99%
72%
✔
✔
✔
✔
10
11
12
No regioselectivity
No chemoselectivity.1-ethyl-4-nitrobenzene, 4-ethyl-aniline and
4-ethenyl-aniline identified
✔
Chemoselectivity. Slow reaction: 95% after 8 h
[a]Reaction conditions: nitroarene (5×10-4 M), NaBH4 (5×10-2 M), MW-PEI@-AuNPs (20 µL), H2O (20 mL), rt; [b]Percentage of substrate transformed within 1 h of
reaction determined by integration of the HPLC peak; [c]Intermediates identified by HPCL-MS.
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were detected whereas adducts resulting from the condensation
route were distinguished for 4-nitrobenzyl alcohol, 4-
nitroacetophenone, 3-nitrostyrene, 4-nitrotoluene, 1-ethynyl-4-
nitrobenzene (Table 3, entries 1, 3, 8, 9, and 11). 4-nitropyridine
(Table 3 entry 7) was transformed but we did not find any match
between the mass spectra and the mass of the intermediates
proposed by Haber.[49] These results support that the reduction
catalyzed by MW-PEI@AuNPs proceeds by both mechanisms,
in full agreement with previous reports for nitrobenzene. Thus,
Corma et al. [50,51] have demonstrated that the reaction proceeds
through the direct route when it is catalyzed by Au-TiO2 while
Noschese et al. [52] showed that the condensation route seems to
prevail when the AuNPs are supported onto a nanoporous
polymeric matrix. Regardless of the reaction route, the reduction
is chemoselective except for 1-ethynyl-4-nitrobenzene (Table 3,
entry 11) that yields not only the expected amine and the
intermediates, but also the reduction of the alkyne group to
alkene and alkane (Figure S5). The interpretation of these data
is not straightforward but they seem to point to the importance of
the position of the substituents, the alkenyl moiety being
reduced when it is placed in para position while remaining
unaltered when placed in meta- (Table 3, entry 8).
agreement with the results reported for AuNPs supported on
commercial silica-PEI beads,[53] the catalytic efficiency
diminishes as the material is reutilized, kapp being 0.55 and 0.16
for cycle 1 and 5 respectively (Figure S7). However, ICP-OES
analyses revealed that the content of gold was 12.1µg/g for the
silica coated with MW-PEI@AuNPs with no significant change
after cycle 5. These results suggest that the decrease of the
catalytic activity is more directly related to the leaching due to
the poor stability of the silica at the alkaline pH of the reaction
rather than to the weakness of the coating.
Standard disposable cuvettes made of polystyrene (PS) and
used in UV-Vis spectroscopy were next considered as a good
model material to gain additional insight into the effect of the
coating on the SPR and the catalytic performance of MW-
PEI@AuNPs since they can be determined in situ. The
adsorption of the MW-PEI@AuNPs turned the surface of the
cuvette reddish (Figure S8) and catalytically active. It is
noteworthy that the reaction proceeds without a detectable
induction time and, although kapp decreases as the cuvette is
reutilized and the reaction proceeds slower, complete reduction
of 4-NP is reached (Figure 4). The analysis of SPR peak reveals
that the adsorption and the reutilization is accompanied by a
red-shift and broadening of the SPR peak. However, changes in
the size of the adsorbed PEI@AuNPs estimated from the
Preparation of catalytic MW-PEI@AuNPs coated materials
Having assessed the catalytic properties, we focused on
exploring whether the affinity of PEI for different materials was
retained by MW-PE@-AuNPs and whether they can coat
surfaces without any additional synthetic step. Our first
candidate was silica, that has been previously used to prepare
supported metal catalysts.[53-57] Since the impregnation of SiO2
with
a gold precursor yields relative large NPs, different
approaches have been successfully assayed to prepare catalytic
composites.[53-57] In addition, commercial silica-PEI beads have
been previously used to synthesize supported AuNPs and the
interaction between PAMAM and SiO2 has been already
exploited to prepare silica-supported gold nanocomposites.[55[
Considering the fact that PEI efficiently coats the surface of
metal oxides such as TiO2, ZrO2, or SnO2 to yield stable amino
functionalized particles that withstand pH 10, we decided to
evaluate the suitability of MW-PEI@AuNPs to functionalize
silica.[25-30] Simple addition of commercial silica to MW-
PEI@AuNPs turned the solution transparent and the silica
reddish almost instantly (Figure S6). Washing with abundant
water and different ionic strengths did not yield any appreciable
leaching and the resulting composite was catalytically active. In
order to put to test the feasibility of using commercial silica
coated with MW-PEI@AuNPs for the large-scale reduction of 4-
NP, 7.5
g
of the nanocomposite were packed in
a
chromatographic column and different volumes of 35-45 mL of
4-NP were passed. The column efficiently reduced up to 920 mL
of solution along 23 cycles with an average conversion rate of
98%. However, when the lag time between two cycles was long
(i.e. hours) the first milliliters eluted from the column (i.e. the
dead volume) showed a faint reddish colour and a weak signal
of the SPR peak at 527 nm was detected confirming the
presence of MW-PEI@AuNPs. This limited leaching was not
observed when there was no stop between cycles and it may be
consequence of the poor long-term stability of silica at alkaline
pH. In order to rule out a desorption event the performance of
0.5 g of silica coated with MW-PEI@AuNPs was evaluated
along 5 cycles of the catalytic reduction of 4-NP. In full
Figure 4. Kinetics of three rounds of reduction of 4NP to 4AP by NaBH4
catalyzed in situ by the MW-PEI@AuNPs immobilized on the surface of a
disposable cuvette (left) and evolution of the SPR peak after each round
(right). Size of the NPs estimated according to expression (1)32 are shown in
brackets.
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Figure 6. Conversion rate in the reduction of 4-NA (blue squares) and 4-NP
(red circles) after a Stain Steel tubing flowing along (1.5 m) coated with MW-
PEI@AuNP
Figure 5. Catalytic performance of different tubing systems (1.5 m) coated
with MW-PEI@AuNP. The tubing was pre-equilibrated with 5 mL of NaBH4
(2.5×10-2 M) (fractions with negative numbering) and then 40 mL of aqueous
an aqueous solution of 4-NP (5×10-5) and NaBH4 (2.5×10-2 M) were passed.
After fraction 19, the reaction mix was supplemented with fresh NaBH4.
Absorbance (400 nm) is normalized by that of the 4-NP solution assayed
(fraction 0)
20 mL of a solution of 4-NP and NaBH4 (Figure S10) revealed
that the tubings were catalytically active, although to at different
extent depending on the composition of the tubing (Figure 5).
FEP was discarded due to its poor performance, that was not
unexpected since as a Teflon®-like polymer this material is used
as anti-adherent coating. After washing thoroughly with distilled
water, the tubings were further tested with 40 mL of a solution of
4-NP and NaBH4 to evaluate the stability of the coating and the
reusability of the tubing. As shown in Figure 5 PEEK, SS and
Tygon® tubings were turned into stable catalysts, the latter
being less efficient, whereas the performance of ETFE was
compromised and it was discarded as a suitable material.
It is well established that NaBH4 decomposes in water[60, 61]
and a closer analysis of Figure 5 shows that performance
dropped as the experiment proceeded and it is improved when
the 4-NP solution was supplemented with fresh NaBH4 after
collecting fraction 19. In order to improve the catalytic efficiency,
the starting solution of 4-NP and NaBH4 was maintained in a
water-ice bath and a new experiment with the PEEK tubing
coated with MW-PEI@AuNPs demonstrated that low
temperature reduces the decomposition of NaBH4, further
supporting the stability and reusability of the coated tubings as
catalysts (Figure S11).
The good performance of the SS tubing and its widespread
use in HPLC encouraged us to further put to test its application.
A new experiment showed that the MW-PEI@AuNPs-coated SS
tubing displayed good catalytic efficiency when assayed with 40
mL of an aqueous solution of NaBH4 and either 4-NA or 4NP, the
average conversion rate being 98% and 97% respectively
(Figure 6) whereas blank experiments with the tubing as
purchased did not show catalytic activity (Figure S12). This
experiment also supports the stability and reusability of the SS
tubing, for which an overall of ca. 340 tubing volumes were
passed through the system without any appreciable loss of
catalytic activity.
spectra are not significantly since they are within the ~11% error
expected from the use of expression (1) (Figure 4).[32]
Polypropylene (PP),
a multipurpose polymer used to
produce plasticware and Falcon®-type and Eppendorf®-type
tubes, was the next material considered. MW-PEI@AuNPs are
also easily adsorbed by this material, being 10 µg/cm2 the
amount of gold attached to the surface. Its catalytic performance
in the reduction of 4-NP was assayed along 11 cycles of reuse.
Although a decrease in kapp is observed during the first cycle,
this parameter is constant from the fourth cycle, demonstrating
that MW-PEI@AuNPs coated PP can be reused a number of
times (Figure S9).
MW-PEI@AuNPs-coated chromatography tubings.
Nanocatalysis has found application in continuous flow
technology in the so-called flow nanocatalysis.[58] The stability of
both NPs and their anchoring to the micro reactor is a critical
issue that has been addressed by three classical approaches: i)
packed beads with the catalyst being tethered onto the bead, ii)
monolithic columns with the catalyst in porous channels resulting
from the copolymerization of the catalyst with other monomers
inside the reactor and iii) wall-coated where the catalyst is
immobilized onto the inner walls of the reactor.[59] In this context,
chromatography tubings are interesting materials since their
functionalization with MW-PEI@AuNPs yields systems that
combine the catalytic activity of the NPs with the high surface
area-to-volume ratio of the tubing. A length of 1.5 m of Tygon®
lab tubing with internal diameter of 0.8 mm and standard
ethylene-tetrafluoroethylene (ETFE), fluorinated ethylene
propylene (FEP), polyetheretherketone (PEEK) and stainless
steel (SS) tubings with internal diameter of 0.5 mm were
selected because they are used in peristaltic pumps and as
tubing systems in liquid chromatography. The immobilization of
the MW-PE@-AuNPs onto the inner surface of the tubing
systems was approached by recirculating the as-prepared MW-
PEI@AuNPs overnight at room temperature. Although no
apparent colour change demonstrating the adsorption of AuNPs
on the inner wall of the tubings was observed, the circulation of
Characterization of the MW-PEI@AuNPs-coated materials
Although the catalytic activity of the coated materials
demonstrated the presence of accessible MW-PEI@AuNPs on
their surface, the samples were further characterized to gain an
additional insight into the coating. Different attempts to prepare
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Table 4. Au 4f binding energies (BE) and FWHM (in parenthesis) of the
Microwave Labstation. UV-Vis spectra were recorded on a Specord Plus
200 spectrophotometer (Analytik Jena AG). IR spectra were recorded in
gold nanoparticles coating different materials
support
BE Au 4f5/2 (eV)
85.5 (2.0)
BE Au 4f7/2 (eV)
82.0 (2.0)
the range 4000-450 cm-1 on
a Spectrum Two (Perkin-Elmer).
Transmission electron microscopy images were obtained from a FEI
TITAN G2 60-300 field emission instrument, equipped with a HAADF
detector at “Centro de Instrumentación Científica”, Universidad de
Granada. Samples were prepared by drying a drop of solution on carbon
coated grid. ICP-OES measurements were carried out on a PERKIN-
ELMER OPTIMA 8300 HPLC-MS. Scanning electron microscopy study
was performed with an Auriga (FIB-FESEM) equipped different detectors
(SE, Se-inLens, BSE, EsB, STEM (Bf/DF) and system for chemical
(EDX) and crystallographic (EBSD) at “Centro de Instrumentación
Científica”, Universidad de Granada. XPS measurements was carried on
a Katros AXIS Ultra-DLD at “Centro de Instrumentación Científica”,
Universidad de Granada. Monochromatic AlKα radiation was used for the
analysis of a surface of 300 x 700 µm of the coated silica and Falcon-
Silica
Falcon®-type tube
PEEK tubing
86.1 (0.8)
82.5 (0.8)
85.6 (1.5)
82.0 (1.5)
thin slices for TEM failed and although the nanoparticles were
not visualized by SEM, EDX analysis revealed the presence of
Au in both large aggregates and along the surface of the
Falcon®-type tube (Figure S13). XPS spectra of the silica, the
Falcon®-type tube and the inner side of the PEEK tubing
showed the Au 4f7/2 and Au 4f5/2 peaks (Figure S14), the binding
energies (BE) differences between Au 4f7/2 and Au 4f5/2 being
close to the standard value of 3.67 eV, indicating the presence
of metallic AuNPs on the surface[62]. The observed Au 4f7/2 BE,
82.0 – 82.5 eV (Table 4), are lower than the expected bulk value
of 84.0 eV but in the range of those reported for supported
bimetallic AuPd nanoparticles.[63] This BE shift of the core levels
of supported AuNPs is well known and it has been related to the
influence of the oxidation (electronic) state of the AuNPs, the
particle size effect or different structural arrangement in the
surface of the nanoparticle.[64] A map of the distribution of Au on
the surface of the Falcon®-type tube was determined by
searching the energy of the Au 4f7/2 a surface of 200 x 200 µm
and further confirmed the coating of the surface (Figure S15).
type tube materials and spot with diameter of 110
µ
m of the
functionalized PEEK tubing, conveniently cut to access to the interior. A
map of the distribution of gold on the surface of the Falcon-type tube, the
intensity of the Au 4f7/2 peak was scanned over a surface of 200 x 200
µm.
Synthesis of the PEI@AuNPs at Room Temperature
HAuCl4·3.5H2O (25 mg, 62.1 μmol) and bPEI2K (50 mg, 100 mg of a
commercial solution) were added to distilled water (50 mL) (PEI
repeating unit:Au ratio =18.7). The synthesis was allowed to proceed for
70 h at room temperature. The process was monitored by UV-Visible
spectrophotometry, recording a spectrum in the range 400-700 nm every
30 min.
Synthesis of the MW-PEI@AuNPs
HAuCl4·3.5H2O (25 mg, 62.1 μmol) and bPEI2K (50 mg, 100 mg of a
commercial solution at 50% w/w) were added to distilled water (50 mL)
(PEI repeating unit:Au ratio =18.7). After homogenization, the solution
was MW irradiated at 300 W and 100 ºC for 3 min. The solution was
allowed to cool to room temperature and the obtained MW-PEI@AuNPs
were used as-prepared without further purification.
Conclusions
bPEI2K is a polyamine suitable for the synthesis of PEI@AuNPs
at room temperature. However their synthesis was optimized by
using a short microwave irradiation that reduces the time of
reaction and yields stabilized AuNPs of suitable size for their use
as a chemoselective catalyst in the NaBH4-mediated reduction
of nitroarenes bearing different functionalities. The role of PEI is
not limited to a reductant and a stabilizer but its known affinity
for different materials has been exploited to coat the surface of
different materials with AuNPs. The anchoring of MW-PEI@NPs
to PS, PP, silica and Tygon®, PEEK and SS tubings yields
novel, effective and stable catalytic materials that can be reused
a number of times. This feature may be of interest in the field of
flow nanocatalysis since PEEK and SS tubings are readily
transformed into very efficient flow reactors by passage of a
solution of MW-PEI@AuNPs without any further synthetic step.
This straightforward functionalization is especially appealing in
the case of the SS tubing due to the widespread application of
this material in HPLC.
Adsorption of MW-PEI@AuNPs onto Silica
Silica Gel 60 (0.063-0.200 mm, 12.5 g) was incubated with the as-
prepared MW-PEI@AuNP solution (25 mL) for 5 min. After this time, the
solution turns colourless and the silica reddish. The solution was filtered
under vacuum and the resulting solid was thoroughly washed with
distilled water and dried under vacuum (1 mm Hg).
Coating of Polystyrene and Polypropylene Surfaces
Standard disposable cuvettes (polystyrene) and Falcon®-type tubes
(polypropylene) were incubated at room temperature overnight with a
solution of as-prepared MW-PEI@-AuNPs. After removing the MW-
PEI@AuNPs solution, the coated materials were thoroughly rinsed with
distilled water and dried at room temperature.
Coating of Tubing with MW-PEI@AuNPs
Experimental Section
The inner wall of 1.5 m of Tygon®, FEP, ETFE, PEEK, and SS tubings
was functionalized by recirculation of the as-prepared MW-PEI@AuNPs
solution at room temperature overnight. After this time each tubing was
washed by circulating distilled water (ca.100 mL).
Materials and Methods
Unless stated otherwise, commercially available compounds where used
without further purification. Masterflex Tygon® lab (formula R-3603, i.d.
0.8 mm), IDEX Health and Science EFTE (Ref. WO 832930, i.d. 0.5
mm), FEP (Ref. WO 828932, i.d. 0.5 mm), PEEK (Ref. WO 721383, i.d.
0.5 mm) and VICI Jour SS (Ref. JR-T-625-20, i.d. 0.5 mm) tubings were
used. Microwave (MW) synthesis was carried out on a Milestone Star
Kinetic Studies of the Reduction of 4-NP, 4-NA and 4-NAc in
Solution
To a solution of the corresponding nitroarene (5×10-5 M) and NaBH4
(2.5×10-2 M) in distilled water (20 mL), a solution of MW-PEI@AuNPs (20
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µL, 1:25 dilution from the as-prepared solution) was added. The reaction
was carried out at room temperature and monitorized by the decrease of
the absorbance at 400 nm, 380 nm and 319 nm resulting from the
reduction of 4-NP, 4-NA and 4-NAc respectively. Between consecutive
measurements the reaction was shaken to favour the elimination of
bubbles. The apparent rate constant was determined from the slope of
the linear correlation of ln(At/Ao) versus time, being At and Ao the
absorbance at the beginning or the reaction (i.e. t=0) and at a given time
(t=t).
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Entry for the Table of Contents
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Mariano Ortega-Muñoz, Victor Blanco,
F. Fernando Hernandez-Mateo, Javier
Lopez-Jaramillo,* and Francisco
Santoyo-Gonzalez*[a]
Page No. – Page No.
Getting Catalytic Gold Surfaces: bPEI2K is a reductant and stabilizer suitable for
the easy preparation of PEI@AuNPs under MW irradiation. The affinity of these
hybrid NPs forthe surface of PS, PP, silica and Tygon®, PEEK and SS tubings has
been exploited for their anchoring yielding effective and stable catalytic materials
Catalytic Materials Based on Surface
Coating by PEI-Stabilized Gold
Nanoparticles
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