Platinum Nanoparticle Impacts at a Liquid|Liquid Interface

Article abstract of DOI:10.1002/anie.201707589

Single nanoparticle (NP) electrochemistry detection at a micro liquid|liquid interface (LLI) is exploited using the catalyzed oxygen reduction reaction (ORR). In this way, current spikes reminiscent of nanoimpacts were recorded, which corresponded to electrocatalytic enhancement of the ORR by Pt NPs. The nature of the LLI allows exploration of new phenomena in single NP electrochemistry. The recorded impacts result from a bipolar reaction occurring at the Pt NP straddling the LLI. O₂ reduction takes place in the aqueous phase, while ferrocene hydride (Fc-H⁺; a complex generated upon facilitated interfacial proton transfer by Fc) is oxidized in the organic phase. Ultimately, the role of reactant partitioning, NP bouncing, or the ability of NPs to induce Marangoni effects, is demonstrated.

Full text of DOI:10.1002/anie.201707589

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Accepted Article
Title: Metal nanoparticle impacts at a liquid|liquid interface
Authors: T. Jane Stockmann, Léo Angelé, Vitor Brasiliense, Catherine
Combellas, and Frédéric Kanoufi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707589
Angew. Chem. 10.1002/ange.201707589
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Metal nanoparticle impacts at a liquid|liquid interface
T. Jane Stockmann,* Léo Angelé, Vitor Brasiliense, Catherine Combellas, Frédéric Kanoufi*
Abstract: Single nanoparticle (NP) electrochemistry detection at a
micro liquid|liquid interface (LLI) is exploited through the oxygen
reduction reaction (ORR) catalysis. In this way, current spikes
reminiscent of nano-impacts were recorded owing to electrocatalytic
enhancement of the ORR by Pt-NPs. The nature of the LLI provides
the exploration of new phenomena in single NP electrochemistry.
The current impacts are due to a bipolar reaction occurring at the Pt-
2
NP straddling the LLI: O reduction takes place in the aqueous
+
phase, while ferrocene hydride (Fc-H ), a complex generated upon
facilitated interfacial proton transfer by Fc, is oxidized in the organic
phase. Ultimately, the role of reactant partitioning, NP bouncing, or
their ability to induce Marangoni effects are evidenced.
The understanding of charge transfer processes at the
nanoscale has been fueled by the emergence of single NP
studies, such as electrochemical nanoimpact experiments,
where insight into the reactivity of a NP is gained from the
electrochemical detection of the collision of individual NPs onto
Scheme 1. Principle of O
sacrificial electron donor at a water|1,2-dichloroethane (w|DCE) interface (Cell
). Electrolytes: H SO in water, P8888TB in DCE.
2
reduction using Pt-NPs and ferrocene (Fc) as a
[
1]
a polarized microelectrode. However such studies, recently
1
2
4
[1a]
[1b]
reviewed by Compton et al., as well as by Robbs and Rees,
are complicated by a rigorous control of the microelectrode
detector surface chemistry and activity. The polarized interface
between two immiscible liquids or electrolytes (ITIES) offers a
more reproducible electrochemical soft-interface. Moreover, the
illustrated in Scheme 1.
For this purpose, a microITIES platform (25μm diameter)
housed at the tip of a pulled borosilicate glass capillary was
use of nanoparticles (NPs) for catalysis at
a polarized
employed. O
using ferrocene (Fc) as a sacrificial electron donor in o, and
SO as the proton source/supporting electrolyte in w.
The system was first analyzed without Pt-NPs and with 5mM
of H SO added to w. In the absence of Fc in o (trace a in Figure
2
reduction is catalyzed by Pt-NPs present in w,
liquid|liquid interface (LLI), between water (w) and oil (o),
particularly for the case of the oxygen reduction reaction (ORR),
H
2
4
[
2]
has gained prevalence.
The ORR is then controlled by
modification of the Galvani potential difference across the ITIES:
2
4
푤
휙푤 − 휙 = ∆ 휙, while NPs enhance the reaction by effectively
표
표
1A), the potential window is limited on either end by the transfer
[
2c,
–
4
+
2ꢀ
4
behaving as multivalent redox species, or electron reservoirs.
of the supporting electrolyte ions: H and HSO or SO
at
3
]
[
11]
Single entity studies with immiscible liquid systems are few.
Ensemble polarographic measurements of TiO , SnO , and
Fe nanoparticles at renewed Hg droplet electrodes were first
positive and negative potentials, respectively. After addition of
0mM Fc to the DCE phase (trace b in Figure 1A), a peak-
2
2
5
2
O
3
shaped wave was observed at –0.150V. As the Fc concentration
[
4]
performed by Heyrovsky et al. Later, the group of Scholz
investigated soft matter microparticle impacts, such as
liposomes and vesicles/organelles, at a Hg electrode. These
were expanded upon using solid ultramicroelectrodes (UMEs) to
investigate nanodroplet impacts. Laborda et al. recently
reported on single emulsion fusion events triggering a large flux
of ions at a macro w|o interface and generating a spike-shaped
푤
was increased the half-wave potential (∆ ϕ1/2) shifted towards
표
more negative potentials while the peak current remained the
[
5]
[6]
[12]
+
same. This is indicative of facilitated ion transfer (FIT) of H by
푤
Fc. Variation in half-wave transfer potential, ∆ 휙
+
with
표
Fcꢀꢁ ,1/2
[7]
[8]
∗
the Fc bulk concentration, 푐 , (Figure 1B) suggests the FIT
produces a (1:1) metallocene-hydride complex Fc-H at a β
value of 4.2×10 L mol . This agrees with recent soft interfacial
Fc
+
1
2
–1
[
1f, 9]
[12c, 12g]
current similar to the destructive impacts of metal-NPs.
However, the study of metal-NP impacts at LLIs only concerns
those at liquid Hg/Pt UMEs or dropping Hg electrodes.
FIT studies.
[
4, 10]
We
propose herein to transpose the concept of electrochemical
nanoimpact to a water|1,2-dichloroethane (w|DCE) soft interface
for the case of the catalytic activation of the ORR by Pt-NPs as
[
a]
Dr. T. Jane Stockmann, Léo Angelé; Vitor Brasiliense, Dr. Catherine
Combellas, Dr. Frédéric Kanoufi
Sorbonne Paris Cité, Paris Diderot University, Interfaces,
Traitements, Organisation et Dynamique des Systèmes, CNRS-
UMR 7086, 15 rue J. A. Baif, 75013 Paris - France.
E-mail: frederic.kanoufi@univ-paris-diderot.fr
Figure 1. (A) Cyclic voltammograms in Cell 1 without (curve a) and with (curve
b) 50mM ferrocene (Fc) with no NP added. (B) Trend of
E-mail: jane.stockmann@univ-paris-diderot.fr
푤
푤
표′
∗
−
zꢂ/(RT)ꢃ∆표 휙
− ∆표 휙 ꢄ versus ln[푐 ].
Fcꢀꢁ,1/2 ꢁ Fc
Supporting information for this article is given via a link at the end of
the document.
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Since Fc can catalyze O
2
reduction itself, although slowly,
is then 퐸표′ = 0.081 V which has a limited driving force at the
푤
the slight negative baseline shift for the red trace in Figure 1A is
applied potential (∆ ϕ >0.2V). It is enhanced with the hydride
표
1/2
+
+
o
likely due to Fc transfer from o→w, which becomes noticeable
Fc-H , an activated form of Fc, which likely has a higher E .
at the high [Fc] used here.
The form of the spikes detected, see inset in Figure 2,
After addition of Pt-NPs, chronoamperograms (CAs) were
recorded at 0.20V, after the FIT wave. Figures 2 A-C depict the
CAs recorded using 5, 30, and 50nm diameter citrate capped Pt-
consists of three regions, as described recently by Stevenson et
[
10a, 10b]
[10c]
al.
Hg
for Pt NPs at a Hg/Pt UME and Compton et al.
Cl NPs at a Hg/carbon UME: (i) a period before impact with
for
2
2
1
1
–1
NPs in w (1.6×10 NP L ); D shows the response with no NPs
added. Once NPs are added, current spikes are observed
reminiscent of electrocatalytic nanoimpacts of Pt-NPs at metallic
no catalysis (baseline),(ii) a sudden onset catalytic current when
the NP collides, and (iii) a seconds-long current decay. The
latter suggests a deactivation of the NP active surface, either by
[
1c-f, 9b, 9c, 13]
+
UMEs.
This suggests the spikes are related to Pt-NP
displacement of the citrate capping agent by Fc , or from the
impacts at the w|o microinterface. Owing to the charge
convention at a LLI, the positive spikes could either be a positive
charge transfer from w→o or a negative one from o→w. The
latter is consistent with electrons passed from o→w, as
illustrated in Scheme 1, where the NP catalyses the ORR in a
bipolar electron transfer mode. Samec et al. electrochemically
synthesized Pt-NPs at a macro w|o interface creating a nanofilm
and proposed a similar interfacial NP catalysed electron transfer
reaction, where decamethylferrocene acts as the electron donor.
action of reactive oxygen species produced during the ORR.
The liquid tube (of height L~150µm and radius a) restricts
NP diffusion toward the LLI. Moreover, this restricted volume,
~23pL, contains (under Figure 2 conditions) ~4 NPs and allows
[14]
the visualisation of single entity behaviour – especially for slowly
diffusing entities. For example, the CA in Figure S1 shows two
sets of multiple peaks: (i) a doublet at 45 and 47s and (ii) 3 large
current spikes at t>82s within 40s. Owing to their uniform
intensity (resp. 150 and 600pA) and residency time (resp. 1 and
2.5s), it can be assumed that each multiplet corresponds to the
same large cluster/aggregate of 30nm diameter NPs bouncing
on the LLI. Such bouncing also likely occurs for individual NPs,
but the knowledge of their diffusion coefficient and concentration,
DNP and cNP respectively, allows predicting the frequency of
impacts f from the equation of NP diffusive flux in the liquid tube
through:
2
푓 = (퐷푁푃푐푁푃휋푎 )/(푎 ꢇ 퐿)
(4)
Using Stokes-Einstein estimates of DNP for the 5, 30 and 50nm
1
1
–1
diameter NPs, respectively, for cNP=1.6×10 NP L in Figure 2,
one predicts ~4, 0.67 and 0.4 current spikes per 100s. Spikes
were selected, based on the above spike profile and excluding
clusters, within a range of 8-100pA. Additional CAs can be found
in the SI. Averaging over ~10 CAs of 120s each, yields 2.5, 1,
and 0.7 impacts per 100s, respectively. The concentration of the
1
1
–1
3
0nm NPs was changed from 1.6 to 7.0 and 16.0×10 NP L
giving f ~2.6 and 8.0 impacts per 100s (3 and 6.7 predicted
resp.). If the observed frequency of impacts is in general
agreement with the predicted flux of NPs to the interface, the
occurrence of frequent spikes, even with lower current amplitude,
detected for the largest 30 and 50 nm NPs (Figure 2B,C) could
indicate NP rolling/bouncing or physical perturbation of the LLI
owing to the presence of the NP. In this way, NPs may instigate
Marangoni-type instabilities, generated by rapid local changes in
surface tension within the back-to-back double layers upon
adsorption.
Figure 2: Chronoamperograms (CA) using Cell 1 (5mM Fc) with a potential
-
11
step from -0.20 to 0.20V with 5, 30, and 50nm diameter Pt-NP (1.6×10 NP L
1
)
for curves A-C, respectively (D: no NP added); inset is a typical current
spike as marked with an *. Arrows: presumed current spikes.
No current spikes were observed at potential steps recorded
before the FIT wave (≤–0.20 V) indicating that here the hydride
+
Fc-H is the electron donor and that the NP catalyses the
combined two interfacial reactions:
ꢂꢅ − H → ꢂꢅ 표ꢆ ꢇ H표 ꢇ 푒ꢀ
ꢆ
ꢆ
Further evidence of Marangoni instabilities induced by NP
(1)
(2)
표
ꢈHꢆ ꢇ O2,푤 ꢇ ꢈ푒ꢀ → H O
electrochemistry
were
sought
through
successive
푤
2
2,푤
positive/negative potential stimulation (large changes in
interfacial tension) of the LLI in the presence of NPs. Figure 3
shows the CA traces recorded at potentials more negative than
the FIT peak (≤–0.20 V). In trace A, obtained before any CA
curve was recorded in the positive region, no current spike was
observed. However, after CAs were performed at positive
potentials (curves B-D), >100pA large negative current spikes
were recorded. This suggests that Pt-NPs may form a film or
The standard redox potential for Fc oxidation in DCE is
[
15]
o
[16]
0
.640 V , while for (2) E = 0.695 V. The standard potential
[
14]
of the combined interfacial redox reaction:
퐸표′ = 퐸표
− 퐸
0
ꢇ ꢋ.ꢋ59pH
(3)
퐹ꢉ+⁄퐹ꢉ
푂ꢊ⁄퐻ꢊ푂ꢊ
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‘
islands’ at the interface upon positive polarization, become
43pA were estimated for the 5, 30, and 50nm NPs respectively,
which are in fair agreement with the experimental values.
deactivated, and then removed at negative potentials. These
data are also reminiscent of recordings by Samec et al.
[14]
via
CV and Pt-NP film electrogenerated at a macroITIES.
Figure 4: Histogram with log-normal trend (solid curve) of current spikes for
(
1
A) 5, (B) 30,–1 and (C) 50 nm diameter Pt-NPs ([NP]
=
1.6, 7.0, and
11
6.0×10 NP L ).
Figure 3 : Chronoamperograms (CAs) using Cell 1 (5mM Fc) with 5nm
11
–1
diameter Pt-NPs dissolved at 1.6 (A and B), 7.0 (C), and 16.0×10 NP L (D)
in the aqueous phase. A was recorded after a cyclic voltammogram was made
to establish the polarizable potential window, but before any further positive
polarization. Curves B-D were recorded after CAs were performed at positive
potentials. CA potential steps were from –0.20 to –0.30V.
The limiting current decreases when a larger portion of the
NP is present in the DCE phase (subhemisphere). In the reverse
situation (superhemisphere) higher currents are expected if, still
2
in a bipolar fashion, O is reduced at the superhemispherical
+
surface, while Fc-H oxidation occurs in the LLI ideally polarized
at the same potential as the lower cap of the NP, present or
Despite the possible aggregate formation, multiple bouncing,
LLI deactivation process (and possible Marangoni effects)
encountered, the observed catalytic current spikes were
discussed more quantitatively. The histograms of their amplitude
are plotted in Figure 4A-C. Even if the presence of NP
contacting the DCE phase. This extreme situation of
nanosphere sitting on the interface provides the highest ilim for
reduction (see [O profiles in Figure S8). Simulated values,
obtained with O partitioning are 6, 37 and 61pA respectively for
a
O
2
2 w
]
2
[
17]
5, 30 and 50nm NPs. This suggests that the smallest NPs are
sitting on the LLI, while the larger ones are likely penetrating the
LLI.
aggregates is suspected,
the smaller current region of each
histogram has been fitted using a log-normal distribution to
establish average spike amplitudes – taken from the log-normal
peak – of 10, 40, and 46pA for the 5, 30 and 50 nm particles,
respectively.
This communication shows a proof of concept towards
liquid|liquid interfacial NP detection/characterization through
stochastic collisions that broadens the number of platforms upon
which this technique can be employed, thereby, expanding the
‘state-of-the-art’. Owing to the non-destructive nature and
flexibility of the LLI, bouncing and/or rolling of NPs was observed,
demonstrating deactivation then re-activation without overall NP
catalytic activity loss. Furthermore, this method provides
sensitive individual as well as ensemble NP information not
possible for nanofilms synthesized in situ at the LLI. Since the
LLI is much easier to fabricate/more reproducible, it could also
replace carbon nanoelectrodes in double-barrel multimode
The observed current amplitudes were rationalized by a
Comsol 2D simulation (details in SI). As presented in Scheme 1,
the model considered the two interfacial reactions (1) and (2)
occurring on each electrolyte side of the NP interface. The
bipolar condition ensures that the reaction is driven with equal
diffusional flux on each NP side. Even though it may depend on
the penetration of the NP in the organic phase (see NP positions
in Figure S7),
a situation explored for solid sphere-cap
[
18]
electrodes resting on a plane, here we explore the situation of
one hemisphere placed in each phase. The simulation predicts
the concentration profiles (Figure S5 for O2,w) and flux (or
equivalent current flow at the ITIES) for the consumption of each
[
19]
sensing
which combines resistive pulse and NP impact
analysis. The fair agreement between the simulated and
observed spike amplitudes, along with the calculated and
experimental impact frequencies, demonstrate the sensitivity of
this method.
2
reactant. Particularly (see Figure S6) the availability of O in the
organic phase and its partitioning, allows local feeding of the
aqueous phase and increase of O2,w mass-transfer at the NP-
water interface. Based on the simulation, the simulated steady-
state current (ilim) is limited by O2,w and values of 4, 26, and
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Detection of nanoparticles (NP) at a
T. Jane Stockmann, Léo Angelé, Vitor
Brasiliense, Catherine Combellas,
Frédéric Kanoufi*
liquid|liquid interface through O
2
reduction and by stochastic impact
events that are recorded
electrochemically. This expands the
detection of metal-NPs to soft
interfaces and provides insight into NP
interaction and the
Page No. – Page No.
Metal nanoparticle impacts at a
liquid|liquid interface
mechanistic/physicochemical
processes at liquid|liquid interfaces.
Layout 2:
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