Single-Molecular Catalysis Identifying Activation Energy of the Intermediate Product and Rate-Limiting Step in Plasmonic Photocatalysis

Article abstract of DOI:10.1021/acs.nanolett.9b05255

Plasmon-mediated photocatalysis provides a novel strategy for harvesting solar energy. Identification of the rate-determining step and its activation energy in plasmon-mediated photocatalysis plays critical roles for understanding the contribution of hot carriers, which facilitates rational designation of catalysts with integrated high photochemical conversion efficiency and catalytic performance. However, it remains a challenge due to a lack of research tools with spatiotemporal resolution that are capable of capturing intermediates. In this work, we used a single-molecule fluorescence approach to investigate a localized surface plasmon resonance (LSPR)-enhanced photocatalytic reaction with subturnover resolution. By introducing variable temperature as an independent parameter in plasmonic photocatalysis, the activation energies of tandem reaction steps, including intermediate generation, product generation, and product desorption, were clearly differentiated, and intermediate generation was found to be the rate-limiting step. Remarkably, the cause of the plasmon-enhanced catalysis performance was found to be its ability of lowering the activation energy of intermediate generation. This study gives new insight into the photochemical energy conversion pathways in plasmon-enhanced photocatalysis and sheds light on designing high-performance plasmonic catalysts.

Full text of DOI:10.1021/acs.nanolett.9b05255

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Communication
Single Molecular Catalysis Identifying Activation Energy of Intermediate
Product and Rate-limiting Step in Plasmonic Photocatalysis
wei li, Junjian MIao, Tianhuan Peng, Hui Lv, Jun-gang Wang, Kun Li, Ying Zhu, and Di Li
Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b05255 • Publication Date (Web): 17 Mar 2020
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Single Molecular Catalysis Identifying Activation Energy of
Intermediate Product and Rate-limiting Step in Plasmonic
Photocatalysis
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Wei Li1,2, Junjian Miao5, Tianhuan Peng1,6, Hui Lv1,2, Jun-Gang Wang3, Kun Li6, Ying Zhu1,4, Di
Li* 3
1 Division of Physical Biology and Bioimaging Center, Shanghai Synchrotron Radiation Facility,
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
2 University of Chinese Academy of Sciences, Beijing 100049, China
3 School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200241,
China
4 Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
5 College of Food Science and Technology, Shanghai Ocean University, Shanghai 201306, China
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State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and
Chemical Engineering, Hunan Provincial Key Laboratory of Biomacromolecular Chemical
Biology, Hunan University, Changsha 410082, China
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ABSTRACT
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Plasmon mediated photocatalysis provides a novel strategy for harvesting solar
energy. Identification of rate determining step and its activation energy in plasmon
mediated photocatalysis plays critical roles for understanding the contribution of hot
carriers, which facilitates rational designation of catalysts with integrated high photo-
chemical conversion efficiency and catalytic performance. However, it remains a
challenge due to a lack of research tools with spatiotemporal resolution that capable of
capturing intermediates. In this work, we used a single molecule fluorescence approach
to investigate a localized surface plasmon resonance (LSPR) enhanced photocatalytic
reaction with sub-turnover resolution. By introducing variable temperature as an
independent parameter in plasmonic photocatalysis, the activation energies of tandem
reaction steps, including intermediate generation, product generation and product
desorption, were clearly differentiated, and intermediates generation was found to be
the rate-limiting step. Remarkably, the cause of plasmon enhanced catalysis
performance was found to be its ability of lowering the activation energy of
intermediates generation. This study gives new insight into the photo-chemical energy
conversion pathways in plasmon enhanced photocatalysis and sheds light on designing
high performance plasmonic catalysts.
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Key words: Plasmonic catalysis, Single molecule fluorescence, activation energy, rate
limiting step,
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Plasmonic photocatalysis by making use of localized surface plasmonic resonance
(LSPR) of noble metal nanoparticles (NPs) has emerged as a promising approach to
facilitate light-driven chemical conversions.1-4 Hot charge carriers, including hot
electrons and hot holes, generated by plasmonic illuminations play synergistic roles in
enhancing catalytic performance.5-7 However, determining activation energies and rate-
limiting step under illumination condition and exploring the contribution of hot carrier
reduced activation barrier is still rare and controversy.1,8 For example, El-Sayed and
coworkers found that the activation barrier for hexacyanoferrate (III) reduction on Au
NPs was slightly higher under visible light excitation than that in the dark, even though
the reaction rate was accelerated under light.9 While for the same reaction, Jain et al.
suggested the activation enthalpy was reduced under light excitation.10 Zhu
demonstrated an appreciable reduction in activation energy when light excitation was
employed for cross-coupling reactions on Au/Pd alloy NPs.11-12 These controversy
indicates elaborated characterizations are needed to obtain precise activation energy of
tandem catalytic steps and to determine rate-limiting step in plasmonic photocatalysis,
which will provide new understandings of the role of hot carriers in plasmon-mediated
photochemistry.
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Single‐molecule fluorescence microscopy (SMFM) has been proven as a powerful
tool in operando investigating elementary chemical reactions on single catalyst.13-17
One unique advantage of single molecular approach in exploring nanocatalysis is its
capability to divide a catalytic turnover into product formation process and desorption
process.18-19 With this sub-turnover resolution, it possesses great potential for obtaining
the thermodynamic and kinetic information of each reaction sequences that are
unavailable from ensemble measurements. Recently, Xu et al. introduced temperature
as an independent variable into single-molecule nanocatalysis, and for the first time
obtained the corresponding activation energies for both product formation and
desorption processes of a fluorogenic reaction.20
Compared with thermal catalysis that involves only chemical transformation,
plasmonic photocatalysis also involves an excitation of hot charge carriers and the
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transmission of hot carriers from plasmonic nanostructures to the adsorbed reactants.21
Thereby, plasmonic photocatalysis contains two or more elementary reactions, and at
least one intermediate was involved.22 As a result, it is more complicated to identify the
rate-limiting step in plasmonic photocatalysis. In the present study, we employed
SMFM to investigate an Au nanorods (NRs) catalyzed reaction with plasmonic
activation. By changing temperature, a temperate-dependent plasmonic photocatalytic
reaction was studied with sub-turnover resolution. Through statistical analysis of single
turnovers, activation energy of rate-limiting step was identified. Remarkably, we found
that the plasmon excitation lowers the activation energy for intermediates generating.
This study exemplifies a new function of single molecule catalysis in exploring the
mechanism of hot carriers enhanced photocatalytic reactions.
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Results and Discussion
Plasmon-Enhanced Au Nanorods Catalyzed Fluorogenic Reaction
A classical plasmonic metal NPs catalyzed fluorogenic oxidation reaction between
nonfluorescent Amplex Red (AR) and H2O2 was chosen as a model to exemplify the
plasmon enhancement. An independent parameter, reaction temperature, was
introduced to obtain the activation energies of tandem reaction steps. The scheme of
experimental setup is shown in Figure 1A. A home-build temperature controllable
microfluidic flow cell equipped with recycling water bath was used as reactor. To
exclude the possible temperature gradient in the channel, two thermocouples were used
to monitor the temperature of water bath and reactor, respectively, in order to ensure
the uniform temperature (Figure S1 and Table S1). Au NRs with aspect ratio of 3.6 and
longitudinal LSPR peak at 785 nm was used as catalysts (Figure 1B). In order to avoid
overlap of LSPR wavelength of Au NPs with excitation wavelength of fluorescent
product Resorufin (Rf, excitation wavelength at 561 nm), Au NRs were adopted as
plasmon photocatalysts rather than Au NPs. A 785 nm laser was introduced to
illuminate the reactor to excite Au NRs.
We first investigated the Au NRs catalyzed fluorogenic reaction at room
temperature under dark or laser illumination. The fluorescent product Rf generated on
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a single Au NR was real-time monitored by total internal reflection fluorescence
microscope (TIRFM) with a temporal resolution of 30 ms/frame (Figure 1D). The
digital fluorescence on−off burst in Figure 1D is a characteristic of single molecule
catalysis. Of note, the fluorescence intensity of on level was not identical, which was
attributed to the presence of multiple catalytic sites on one Au NR or accumulated
products stay on one catalytic site before desorption. Moreover, several control
experiments were performed to confirm the occurring of the single molecule catalysis
on single nanocatalyst (part 3 in supporting information). These include: (1) no
stochastic fluorescence bursts were observed in the absence of either Au NRs or
reaction substrates (AR and H2O2), indicating the fluorescence burst is a result of
catalytic reaction (Figure S2); (2) no digital fluorescence bursts were observed when
Rf solution was flowed over Au NRs under the same experiment condition, suggesting
the fluorescence burst is not stem from binding/unbinding of free Rf to Au NRs (Figure
S3); (3) the fluorescence burst frequencies were independent of 561 nm laser intensity
(Figure S4); (4) the average time scale of fluorescence on is less than 0.2 s, which is
much shorter than the average photobleaching lifetime of Rf molecule (ca. 25 s) under
similar laser intensity.13 Therefore, we convince ourselves that each sudden
fluorescence burst represents the generation of a Rf molecule on single Au NR.
In these single-molecule fluorescence trajectories, photocatalytic events contain
two stochastic waiting times, τoff and τon, representing the time before each Rf formation
and then desorption from Au NRs, respectively. The inverse of τoff and τon thus represent
the reaction rate for product generation and desorption. Interestingly, under 785 nm
laser illumination, the appearance frequency of fluorescence burst was increased
(Figure 1D and Figure S5). We calculated the average <τoff>-1 and <τon>-1 (< > denotes
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average) from 50 trajectories under dark or laser illumination, respectively. Under 785
-1
nm laser illumination, the average product generation rate (<τ
> ) was calculated
off, light
to be 2.1 ± 0.4 s-1, which was 3 times faster than that under the dark (<τoff, dark>-1, 0.7 ±
0.3 s-1) (Figure 1E). While for the product desorption rate, <τon, light>-1 (10.3 ± 0.9 s-1)
was also about 1.3 times faster than <τon, dark>-1 (8.1 ± 0.8 s-1) (Figure 1E). The
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comparison of single molecule product generation and desorption rate clearly supported
the plasmonic enhancement. In addition, we also compared the catalytic reaction of
bulk solution under dark or laser illumination, and found similar plasmonic enhanced
performance (part 4 and Figure S6 in supporting information), further confirming the
solidity of single molecular measurements.
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Figure 1. Plasmon enhanced photocatalysis on single Au NR. (A) Schematic illustrating the
individual Au NRs catalyzed fluorogenic reaction under 785 nm laser illumination with temperature
control setup. (B) TEM image of Au NRs, inset: UV-vis spectrum of Au NRs. (C) Typical TIRFM
images of single Rf molecule generated on single Au NRs under dark (top) or laser illumination
(bottom) at 298 K in the presence of 400 nM AR and 88.3 mM H2O2. (D) Typical fluorescence
trajectories recorded from the fluorescent spots marked with arrows in (C). (E) Comparison of
<τoff>-1 (left) and <τon>-1 (right) derived from fluorescence trajectories under dark or laser
illumination. Solid lines are fit with Gaussian function.
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Single Molecular Measurements Identify Intermediate and Rate-Limiting Step
under Dark
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Having confirmed the Au NRs catalyzed fluorogenic reaction could be enhanced
by plasmon, we then attempted to capture intermediate products by harnessing the
single molecular kinetical analysis. As shown in Figure 1E and F, the product generation
rate (<τoff>-1) is much smaller than its desorption rate (<τon>-1) under either dark or laser
illumination, indicating that rate-limiting of this reaction is involved in the product
generation step.
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The reaction mechanism between AR and H2O2 has been extensively studied.23-24
Substrate AR is first oxidized to a nonfluorescent intermediate product AR⋅ and then
converted into final fluorescent product Rf with reaction rate constants of k1 and k2,
respectively (Figure 2A). In order to further support the multi-step reaction mechanism,
we introduced DMSO into substrate as OH⋅ scavenger, and found the fluorescence burst
was gradually disappeared (part 5 and Figure S7 in supporting information). This
suggested that OH⋅ is indeed involved and confirmed the pathway in Figure 2A.
However, kinetics of individual step in Figure 2A is unavailable from ensemble
measurements and thereby hard to identify the rate-limiting step. τoff in fluorescence
trajectory involves the reaction time required for the oxidation of AR to AR⋅, and then
to Rf. Of note, H2O2 was kept large excess, thus the reaction can be considered as a
quasi-first order reaction of AR. Figure 2B shows the distribution of τoff from a single
Au NR (more data collected from other AR concentrations in Figure S8). Obviously,
the distribution of τoff follows a quick rise and then gradual decay. This rise and decay
distribution of τoff indicated that the formation the fluorescent product contains at least
two sequential steps and a hidden kinetic intermediate,24-25 which coincides with
previous mechanism explorations.26 To further verify the kinetic intermediate, we
investigated a control N-deoxygenation reaction of Resazurin (Rz) to Resorufin (Figure
2C). This reaction mechanism is identified as a single step reduction.27 In sharp contrast
to the τoff distribution shown in Figure 2B, τoff of the deoxygenation reaction follows a
single exponential decay distribution (Figure 2D). Therefore, we concluded the rise-
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decay behavior of τoff in Figure 2B indeed contains at least two sequential reaction steps.
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Figure 2. Identification the rate-limiting step of the fluorogenic reaction under dark. (A) Reaction
mechanism of oxidative deacetylation of Amplex Red to Resorufin. (B) Distribution of τoff, dark for
the oxidative deacetylation reaction from a single Au NR in the presence of 400 nM AR and 88.3
mM H2O2. Red lines are empirical fit with ꢀ = ꢁ(ꢂꢃꢄ ꢆ − ꢂꢃꢄ ꢆ) (a simplified form of Eq. 1).
(C) Reaction mechanism of reductive N-deoxygenation of Resazurin to Resorufin. (D) Distribution
of τoff, dark for the reductive N-deoxygenation reaction from a single Au NR in the presence of 50 nM
Rz and 180 μM NH2OH. Red lines are empirical fit with ꢀ = ꢁꢂꢃꢄꢆ. (E) and (F) Dependence of
the apparent rate constant k1(E) and k2 (F) on [AR] from the same Au NRs.
ꢅ
ꢇ
We next investigated the single molecule kinetics of each step to identify rate-
limiting step. Based on noncompetitive Langmuir−Hinshelwood mechanism, the
probability density function f(τ) of τoff can be derived as:
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ꢄ ꢄ
ꢅ
ꢇ
ꢈ(ꢉ) =
(ꢂꢃꢄ ꢆ − ꢂꢃꢄ ꢆ
)
(1)
ꢅ
ꢇ
ꢄ ꢃꢄ
ꢇ
ꢅ
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where k1 and k2 are the corresponding apparent rate constants of intermediate product
formation and final product formation (Figure 2A), respectively.24 k1 and k2 can take
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the following forms when H2O2 concentration was kept large excess:
[ꢌ]→ꢍ
[ ] (
)
ꢊꢋ ꢎ⎯⎯⎯ꢏ ꢐꢑꢒꢒꢋꢓꢔ ꢁ / 1 + ꢓꢔ[ꢁ]
(2)
(3)
[ꢌ]→ꢍ
ꢊꢕ ꢎ⎯⎯⎯ꢏ ꢐꢑꢒꢒꢕ
where GA is the adsorption equilibrium constant of AR, γeff1 and γeff2 are the effective
rate constants of intermediate product formation and final product formation,
respectively, [A] represents the concentration of the substrate AR, [H] represents the
concentration of H2O2.24
We then analyzed multiple fluorescence trajectories to derive k1 and k2 (Figure 2E
and F), each point in Figure 2E and F was averaged over multiple fluorescence
trajectories and the solid lines were fits with Eq. 2 or 3. We found that k1 was dependent
on [AR] and reached plateau when [AR] was higher than 400 nM (Figure 2E), while k2
was independent of [AR] and remained unchanged (Figure 2F). The calculated k2 was
one order of magnitude higher than k1, indicating that AR⋅ formation is the rate-limiting
step in the product generation.
Identification of the Activation Energy of Rate-limiting Step in Plasmonic
Photocatalysis
The apparent activation energy of each reaction step (Ea,i) could be obtained from
the Arrhenius equation;
⁄
RT
ꢊꢖ = ꢁꢖꢂꢃꢗ
(4)
ꢘ,ꢙ
where ki is the rate constant and Ai is the preexponential factors of step i.
Taking into consideration that k1 and k2 could be simplified as k1 → γeff1, k2 → γeff2
when [AR] = 400 nM and H2O2 was large excess, Eq. 4 could be deduced to:
( )
(
)
(
)
⁄
ln ꢊꢋ = ln ꢐeff1 = ln ꢁoff1 − ꢚꢛ, off1 RT (5)
( )
(
)
(
)
⁄
ln ꢊꢕ = ln ꢐeff2 = ln ꢁoff2 − ꢚꢛ, off2 RT
(6)
For final product desorption process, k3 → <τon>-1 when [AR] = 400 nM and H2O2
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was large excess (for detailed derivations, see part 7 in Supporting Information),
therefore
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( )
ln ꢊꢜ = ln ꢉꢝꢞ
(〈 〉ꢃꢋ)
(
)
⁄
= ln ꢁꢝꢞ − ꢚꢛ, on RT (7)
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where Aoff and Aon represent the preexponential factor of the surface process occurred
in τoff and τon, respectively, Ea,off1, Ea,off2 and Ea,on are the activation energy for
intermediate product formation process, final product formation and final product
desorption process, respectively, k3 is the rate constant of final product desorption.
We then monitored the temperature-dependent catalytic reaction under dark or 785
nm laser illumination to derive the activation energies of different tandem reaction steps.
By extracting τoff in each trajectory, the effective rate constants γeff1 and γeff2 at different
temperature could be derived by fitting the distributions of τoff with eq. 1 (Table S2).
Therefore, the activation energy for intermediate formation (<Ea, off1>), final product
formation (<Ea, off2>) and product desorption process (<Ea, on>) were derived from Eq.
5-7 and demonstrated in Figure 3C and Table 1.
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Table 1. Activation Energies of Tandem Reaction Steps at Dark or under Laser
Illumination
<Ea, off1
>
<Ea, off2
>
<Ea,on>
(kJ mol-1)
52.9 ± 3.8
42.4 ± 1.6
(kJ mol-1)
5.5 ± 1.4
5.3 ± 1.3
(kJ mol-1)
21.9 ± 3.2
21.4 ± 2.8
Under the dark
Under illumination
Interestingly, the activation energy for intermediate formation is significantly
higher than that of Rf formation, further confirming the generation of AR⋅ with higher
energy barrier is the rate-limiting step of this fluorogenic reaction. Under illumination
with 785 nm laser, activation energy of this step significantly decreased down to
42.4±1.6 kJ mol-1, while the activation energies of final product generation and
desorption remained barely unchanged. The reduced energy barrier of rate-limiting step
might be originated from the plasmonic facilitated generation of OH⋅ from H2O2.28
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Figure 3. Identification the activation energies of tandem reaction steps during in plasmon
photocatalytic reaction. Distributions of τoff under dark (A) or laser illumination (B) from single Au
NR at different temperatures in the presence of 400 nM AR and 83.3 mM H2O2. Solid red lines are
empirical fits of Eq 1. (C) The distribution of Ea i for intermediate formation process (top), final
product formation process (medium) and product desorption process (bottom) calculated from over
100 Au NRs. Insets: the average Arrhenius plots under dark (black rectangle, ) or laser illumination
(red circle, ○) over 100 Au NRs.
To further confirm the plasmon lowered Ea of intermediate, we interrogated
reductive N-deoxygenation of Resazurin to Resorufin under 785 nm laser illumination.
In contrast to the plasmon enhancement, we observed negligible change in catalytic
performance (Figure S9). These results clearly indicated that the plasmon enhanced
catalytic performance originates from its ability of lowering the activation energy of
rate-limiting step.
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Compensation and Isokinetic Effect Interpreting the Role of Hot Carrier for the
Enhanced Catalysis
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Now, it is obvious that laser illumination accelerates the reaction rate by lowering
the activation energy of intermediate product formation. A subsequent challenge is to
reveal the contribution of hot carriers to the lowered activation energy.
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Multiple possibilities have been proposed to explain the plasmon enhanced
catalysis, including plasmon resulted local heating of reaction surroundings
(photothermal effect)29-31, plasmon-induced changes of nanocatalysts itself
(morphology and active site rearrangement)32-33 and hot carriers facilitated reactant-
catalyst interaction.34-36 Particularly, thermal effect stemming from plasmon decays is
crucial for most plasmonic applications. However, quantifying thermal effects remains
extremely challenging due to the experimental difficulty in accurately measuring the
surface temperature of plasmonic nanostructures.37 To solve this problem, we harnessed
several parameters, including weak laser intensity and short Au NRs, to minimize the
contribution from thermal effect to extrude the role of hot carriers (see part 9 in
supporting information. We carried out several control simulations to calculate the
surface temperature of Au NRs during laser irradiation (Figure S10 in supporting
information), and found negligible temperature and morphology change. The
morphology of Au NRs was also examined after the plasmonic photocatalysis (part 10
and Figure S11 in supporting information), and negligible morphology change was
observed, thus possible contributions from photothermal effect and morphology were
ruled out. In addition, the surface re-arrangement of catalyst is closely correlated with
its fluctuations of activity, we thereby calculated the temporal fluctuations of activity
of Au NRs under laser irradiation and found ignorable changes (Figure S12).13
Therefore, we speculated the contribution of plasmon enhancement is not stem from
the catalyst itself, but from the improved interactions between catalysts and substrates.
In transition state theory, activation energy of a chemical reaction is relevant to the
saddle point of potential energy surface, and in some cases its value depends on the
vibrational energy stored in the bond to a great deal.38-40 The vibration frequency of a
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specific bond could be reflected by isokinetic temperature (Tiso) in isokinetic effect,
which is related to, but not identical with compensation effect (a linear correlation
between activation energy (Ea, which determines the temperature dependence) and
frequency factor (A) in the Arrhenius dependence).41-42 Compensation and isokinetic
effect have been extensively observed in many thermal activated heterogeneous
catalytic reactions, and are considered to involve valuable information about the
enthalpy and entropy changes of a reaction.43-44 The validity of compensation and
isokinetic effect at the single particle level has been demonstrated by Xu et al,20
however, its validity in photocatalysis reaction is still uncertain.
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Exner had suggested that when T1/T2 ≈ 1, the compensation effect could be more
reliably checked by plotting lnki,1 vs lnki,2.45 In the present product formation process,
the rate constant ki represents the effective rate constants of intermediate product
formation. ki,1 and ki,2 are the values of the rate constant ki at two random temperatures
T1 and T2, respectively (T1>T2). In both dark reaction and laser illumination, lnγeff1,1
was positively correlated to lnγeff1,2 (Figure 4A and B), indicating that the activation
parameters indeed compensate one another and the compensation effect is also
applicable to photocatalysis.
To further confirm the lower activation energy by plasmon exciting, the isokinetic
(or isoequilibrium) temperature (Tiso), the special temperature that all the reactions in
the series should have same rate (or equilibrium) constant, have also been investigated.
We divided the individual gold nanorod into three subgroups according to the values of
the activation energies. The dividing subgroups of individual gold nanorod were based
on the following: for dark reaction small group: 44 ≤ Ea, off1<49 kJ mol-1; middle group:
49 ≤ Ea, off1 <54 kJ mol-1; large group: 54 ≤ Ea, off1<59 kJ mol-1. For laser illumination,
small group: 39 ≤ Ea, off1<42 kJ mol-1; middle group: 42 ≤ Ea, off1<45 kJ mol-1; large
group: 45≤ Ea, off1<48 kJ mol-1. For each group, lnγeff1 (where the rate constant γeff1 is
an average over the entire subgroup) was plotted as a function of 1000/T. As shown in
Figure 4C and D, for both dark reaction and under laser illumination, three independent
plots intersect at one point. This indicates that an isokinetic relationship exists in
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photocatalysis. Compared to dark reaction (Tiso= 316 K), the Tiso for laser illumination
(Tiso=314 K) reduced 2K, which probably indicates the substrate with lower energy
barrier to convert into intermediate under laser illumination. We thus speculated that
the plasmon excited electrons can induce multiple vibrational transitions of the Au-O
bond that increases vibrational energy stored in the bond (Figure S13).1 As a result, Tiso
was reduced owing to the lowered activation energy.
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Figure 4 Compensation and isokinetic effect in the single-molecule photocatalysis of individual Au
NR. Dependence of kinetic rate constants on individual Au NR at two temperatures for dark reaction
and laser illumination (T1, T2, T1 > T2, here, T1: {303 K, 308 K; 313 K}; T2: {298 K, 303 K; 308
K}). Red lines are linear fits with R2 = 0.61 (A) and R2 = 0.66 (B). Isokinetic relationship of three
groups of single gold nanorod with different average Ea, off1 in the coordinates 1000/T and lnγeff1 for
dark reaction (C) and laser illumination (D). Three groups for (C): small group: 44 ≤ Ea, off1 < 49 kJ
mol-1 (blue); middle group: 49 ≤ Ea, off1 < 54 kJ mol-1 (red); large group: 54 ≤ Ea, off1 < 59 kJ mol-1
(black). Three groups for (D): small group: 39 ≤ Ea, off1 < 42 kJ mol-1 (blue); middle group: 42 ≤ Ea,
off1 < 45 kJ mol-1 (red); large group: 45 ≤ Ea, off1 < 48 kJ mol-1 (black). The solid lines are the linear
fittings.
Conclusions
In summary, we have proposed a single molecular strategy to derive rate-limiting
step and activation energy of a plasmonic catalysis reaction by monitoring the
temperature-dependent catalytic activity. The reason of plasmon enhancement was
attributed to its ability to lower Ea of the rate-limiting step. Of note in the present
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fluorogenic reaction example, the rate-limiting step is the intermediate product
generation, however, the strategy could be expanded to other plasmon enhanced
reactions to derive rate-limiting steps. This ability provides new insight into exploring
the mechanism of plasmonic catalysis, which deepens our understanding of plasmonic
reaction and facilities designing desirable plasmon catalysts.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website
at DOI:xxxxxxxxxx.
Experimental section and all control experiments availability.
AUTHOR INFORMATION
Corresponding Author：*E-mail: dli@chem.ecnu.edu.cn
ORCID
Di Li: 0000-0003-1674-0110
Notes
The authors declare no competing financial interest
ACKNOWLEDGMENTS
This work was supported by NSFC (21675166, 21874046, 21902050) and National
Postdoctoral Program for Innovative Talents (BX20190118). The Shanghai Municipal
Commission for Science and Technology (19JC1411800). We thank Prof. Phillip
Christopher for helpful discussion.
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