Artificial photosynthetic assemblies constructed by the self-assembly of synthetic building blocks for enhanced photocatalytic hydrogen evolution

Article abstract of DOI:10.1039/d0ta08325a

An artificial photosynthetic assembly (APA) of a hollow-rod structure was successfully constructed by using synthetic building blocks to mimic the structure and function of natural photosynthetic bacteria. The APA was formed by the incorporation of carbon nanoparticles as light harvesters into an enzyme-like polymer, PEI-Co, containing cobalt complexes as redox catalytic centres. The APA features a bacteria-like shape of ca. 2-3 μm length rods and a hollow structure positioning photosynthetic components at the surface. The APA integrates key components, the light harvester, redox catalyst, and proton relay group, of photosynthetic systems in assemblies formed from a polymeric framework. The APA system in aqueous solution converts protons to H2 under visible light irradiation with obvious advantages. It exhibits a 50-fold improvement in hydrogen production activity and has a broader pH response of photocatalytic H2 production compared with a non-assembled system. This journal is

Full text of DOI:10.1039/d0ta08325a

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W. Xia, M. Lan, X. Xing, J. Hu, L. Huang, J. Liu, Y. Ren and H. Liu, J. Mater. Chem. A, 2020, DOI:
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28 March 2018
Pages 4883-5230
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Artificial Photosynthetic Assemblies Constructed by the
Self-Assembly of Synthetic Building Blocks for Enhanced
Photocatalytic Hydrogen Evolution
Jin Wu,a Wu Xia,a Minhuan Lan,b Xue-Jian Xing,b Jun-Chao Hu,a Li Huang,b Jing Liu,a Ying-Yi
Ren,a Hongfang Liu,a Feng Wang*a
a.
Key Laboratory of Materials Chemistry for Energy Conversion and Storage (Huazhong
University of Science and Technology) of Ministry of Education, Hubei Key Laboratory of
Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering,
Huazhong University of Science and Technology, Wuhan, 430074 (P. R. China), Email:
wangfengchem@hust.edu.cn
b.
Hunan Provincial Key Laboratory of Micro & Nano Materials Interface Science, College of
Chemistry and Chemical Engineering, Central South University, Changsha, 410083 (P. R.
China)
Keywords: self-assembly, artificial photosynthesis, hydrogen production, biomimic, carbon
nanoparticles
Abstract: An artificial photosynthetic assemblies (APA) of hollow-rod structure was
successfully constructed by using synthetic building blocks to mimic the structure and function
of natural photosynthetic bacteria. The APA was formed by the self-assembly of incorporation of
carbon nanoparticles as light harvesters into an enzyme-like polymer, PEI-Co, containing cobalt
complexes as redox catalytic centres. The APA features a bacteria-like shape of ca. 2-3 μm
length rod and a hollow structure of positioning photosynthetic components at the surface. The
1

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APA integrates key components, light harvester, redox catalyst, and proton relay group, of
photosynthetic system in assemblies formed by polymeric framework. The APA system in
aqueous solution converts proton to H2 under visible light irradiation with obvious advantages. It
exhibits a 50-fold improvement on hydrogen production activity and have a broader pH response
of photocatalytic H2 production compared with those of a non-assembled system.
1. Introduction
Self-assembly of synthetic building blocks to rebuild biomaterials and biomimics is of
considerable interest to chemists.1-5 In this sense, some morphological or functional mimics, such
as artificial hierarchical structures,6 enzymes,7-9 and cells,10-12 have been fabricated in recent
years. An ideal biomimic should have both morphological and functional characteristics like
those of the target. However, achievement of this goal remains a challenging task, and successful
examples are rarely documented.
Natural photosynthetic bacteria (NPB) are single-cell organisms capable of converting organic
acid to hydrogen (H2) via a photofermentation process under visible light irradiation.13, 14 This
property makes NPB useful in industrial and agricultural effluent treatment for obtaining H2 as
renewable energy. The photosynthetic H2 evolution of NPB occurs in photosynthetic systems,
which are located at the cell membrane to maximize light absorption.15, 16 In a photosynthetic
system, the protein-binding light-harvesting pigments and redox catalytic centres form
microenvironments, in which several light harvesting pigments surround a redox catalytic
centre.15, 16 During photosynthesis, the pigments deliver excited electrons to a nearby redox
reaction centre to generate a reduced species, which attracts protons and releases hydrogen. The
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appropriate location and arrangement of photosynthetic systems in NPB ensures maximum light
absorption by the pigments and efficient photoinduced electron transfer (PET) and protonation.
Herein, we wish to report the successful construction of a kind of artificial photosynthetic
assemblies (APA), which is self-assembled from synthetic components. The APA features a
bacteria-like shape and structure and can produce H2 under visible light irradiation. A synthetic
enzyme PEI-Co (Scheme 1) was obtained by grafting the [Co(TPA)Cl2] redox reaction centres
onto branched polyethylenimine (PEI), and surface-functionalized carbon nanoparticles (CNPs)
were synthesized. Photoluminescent carbon nanoparticles are a kind of emerging emissive
materials with advantages of low cost, easy preparation, and free of toxic metals, which had been
used in bioimaging, sensing, and light emitting devices.17-20 However, the utilization of CNPs as
a photosensitizer for solar fuels production is rare.21 PEI-Co and CNPs can self-assemble in an
aqueous solution to form uniform hollow rod-like CNPs@PEI-Co assemblies (APA) ca. 2-3 μm
in length. In these assemblies, CNPs act as a light harvester located at the surface of the hollow
rod structures formed by branched polymeric PEI-Co. The self-assembled APA have enhanced
activity and broader pH response of the photocatalytic H2 production compared with those of a
non-assembled system.22-27 The APA system efficiently produced H2 within a pH range of 4.0-
10.0; however, the non-assembled system was functional only within a pH range of 2.0-5.0; the
APA system has 50-fold higher turnover number (TON) of H2 production compared with that of
the non-assembled system. At the optimal conditions, the APA system produced H2 with a
turnover number (TON, based on cobalt active sites in PEI-Co) up to 198 and an initial rate of
538 μmol g-1 h-1 (based on mass of CNPs@PEI-Co assemblies), these values exceed those of
state-of-the-art homogeneous photocatalytic H2 production systems using carbon dots and cobalt
complexes.28-30
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2. Results and discussion
2.1 Synthesis and characterization of PEI-Co
Scheme 1. The synthetic routes and chemical structures of organic ligand L1-L3, complex C1
and PEI-Co (NBS: N-bromosuccinimide, BPO: benzoyl peroxide, DPA: 2,2-dipicolylamine,
PyBOP: benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate; details of the
synthesis are given in Experimental section).
The PEI-Co was designed by grafting the carboxyl group modified cobalt complex C1 onto
branched PEI (Mw = 25 k) by an amide bond (Scheme 1 and Experimental section).31 The
branched PEI was selected for three reasons. First, its branched polymeric structure can mimic
protein environment.32, 33 Second, the abundant amino groups of PEI act as a protons relay for
efficient protonation of the catalytic centre.34-36 Third, these positively charged amino groups
function as binding sites to attract negatively charged CNPs to form assemblies. The successful
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Figure 1. (a) UV-Vis absorption spectra of PEI-Co (0.10 mg mL-1, 8.46×10-6 M), PEI (0.10 mg
mL-1), and C1 (5.29×10-4 M in aqueous solution. (b) IR spectra of PEI-Co, PEI, and C1.
grafting of C1 onto PEI was characterized by UV-Vis and IR spectroscopy. The UV-Vis
spectrum (Fig. 1a) of PEI-Co shows characteristic TPA-ligand absorption at 260 nm, which is
also observed in the spectrum of C1. Broad weak absorption at 330 - 560 nm in C1 is assigned to
LMCT absorption of cobalt core;37, 38 this absorption was suppressed due to low concentration of
C1 in PEI-Co. IR spectrum of PEI-Co (Fig. 1b) shows an amide signal at 1658 cm-1, which is
different from the characteristic C=O signal at 1734 cm-1 of the carboxyl group of C1.39 The
amount of C1 in PEI-Co was 8.46 × 10-5 mmol mg-1 as determined by inductively coupled
plasma-atomic emission spectrometry (ICP-AES). Another two samples with different C1
loading amount, PEI-Co* of [Co] = 2.88 × 10-6 M and PEI-Co** of [Co] = 5.24 × 10-5 M, were
prepared for comparison experiments. The surface carboxyl groups-functionalized CNPs were
prepared by hydrothermal treatment using poly-3-thiopheneacetic acid in NaOH aqueous
solution as characterized and reported previously.40 The CNPs used in this work were
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nanoparticles with a diameter of 20-60 nm (SI, Fig. S1). The UV-Vis spectrum of the CNPs have
a strong absorption in the range of 300-600 nm with a maximum at 415 nm (SI, Fig. S4).
2.2 Self-assembly of APA
The self-assembly of PEI-Co and CNPs was performed in water. We investigated the
morphology of individual PEI-Co and CNPs by SEM and confocal laser scanning fluorescence
microscopy. The SEM image (Fig. 2d) shows that PEI-Co uniformly forms ca. 2.0-3.0 μm long
hollow rod structures. Mixing PEI-Co with CNPs results in CNPs incorporation into the PEI-Co
assemblies to form the CNPs@PEI-Co assemblies. As shown in SEM image (Fig. 2e), the
CNPs@PEI-Co assemblies maintain ca. 2.0-3.0 μm long rod structure; however, they have a
nodular morphology different from that of PEI-Co, indicating the incorporation of CNPs into
PEI-Co. The CNPs@PEI-Co assemblies are emissive under 405 nm laser excitation in the green
confocal channel and merged images (Fig. 2b and 2c). Interestingly, the emission areas are
concentrated on the edges of the rod-shaped assemblies, indicating that the CNPs locate at the
surface of the hollow PEI-Co assemblies. The special structure of PEI-Co and CNPs@PEI-Co
assemblies in water may be caused by the amphiphilic property of the branched PEI-Co. Namely,
the positively charged amino groups (discussed below) and the organic -CH2CH2- framework in
PEI-Co act as a hydrophilic and hydrophobic part, respectively. The positively charged amino
groups in PEI-Co stay outside of the hollow structures; they act as binding sites to attract
negatively charged CNPs on the surface of the hollow assemblies. This self-assembly process
was continuously monitored by confocal microcopy and recorded as a video (SI). The whole
process was recorded starting from the addition of CNPs into aqueous solution of PEI-Co to the
formation of the emissive CNPs@PEI-Co assemblies (Fig. 2f-i). The non-fluorescent rod
structures of PEI-Co move in solution prior to the addition of CNPs (SI, Fig. S5 and S6). After
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Figure 2. (a)-(c) Confocal microscopy images of CNPs@PEI-Co, inset: enlarged view of the
selected area (red line). (d) SEM image of PEI-Co. (e) SEM image of CNPs@PEI-Co. (f)-(i)
Confocal microscopy images recorded after adding CNPs into aqueous solution of PEI-Co. (j)
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Surface zeta-potentials of PEI-Co and CNPs in aqueous solution at different pH values. (k)
Surface zeta-potentials of PEI-Co, CNPs, and CNPs@PEI-Co in aqueous solution at pH 8.4. (l)
FTIR second derivative spectra of CNPs@PEI-Co at different pH values. (m) Schematic
diagram of the self-assembly of PEI-Co and CNPs.
adding CNPs to the solution (Fig. 2f), no fluorescence was observed at the initial stage indicating
that fluorescence of CNPs in its mono-dispersion state is unobservable under these conditions.
However, a few emissive assemblies were formed after approximately one minute (Fig. 2g). This
result indicates that CNPs are incorporated into the PEI-Co assemblies, and the fluorescence of
CNPs can be observed after CNPs@PEI-Co formation. In the next recording time, additional
fluorescent CNPs@PEI-Co assemblies can be observed in confocal images (Fig. 2h-i). Two
synthetic building blocks, PEI-Co and CNPs, were successfully self-assembled into the
CNPs@PEI-Co assemblies; in these assemblies, the light harvester CNPs and the redox catalytic
cobalt complexes compose the photosynthetic systems confined in the microenvironment formed
by the PEI framework and are located at the surface of the assemblies, which resembles the
situation in NPB.15, 16
To determine the driving force of this self-assembly, surface zeta-potential measurements were
performed. The relationship between the surface zeta-potential (ξ) and pH values of PEI-Co and
CNPs in aqueous solution are shown in Fig. 2j. The ξ values of PEI-Co are positive in the pH
range of 3.0-12.0, indicating that PEI-Co is positively charged on the surface.41 Higher surface
zeta-potentials were obtained in acidic solutions increasing from 4.35 mV at pH 9.2 to 24.7 mV
at pH 3.0. This increase is due to the protonation of amino groups of PEI-Co. The CNPs
containing surface carboxylic acid groups are negatively charged in aqueous solution; thus,
higher ξ values were obtained in basic solutions. The ξ values increased from -14.97 mV at pH
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5.4 to -52.63 mV at pH 12.4 because of deprotonation of the surface carboxylic acid groups of
CNPs under the basic conditions.42 Mixing PEI-Co solution (0.10 mg mL-1) with a ξ value of
13.4 mV and CNPs solution (0.14 mg mL-1) with a ξ value of -28.8 mV at an equal volume ratio
results in a ξ value of -24.2 mV of the self-assembled CNPs@PEI-Co (Fig. 2k). Therefore, the
driving force of the self-assembly can be attributed to electrostatic interactions between the
positively charged PEI-Co and negatively charged CNPs.26, 43-45 The electrostatic interaction in
CNPs@PEI-Co was further confirmed by the second derivative spectra of the corresponding
peaks of the FTIR spectra (Fig. 2l). The C=O stretching vibration peak of the carboxyl groups of
CNPs is moved from 1656 cm-1 assigned to -COOH at pH 5.25 to 1651 cm-1 assigned to
deprotonated -COO- at pH 9.72. Analogous changes in the N-H bending vibration peak in PEI-
+
Co from 1583 cm-1 of -NH3 in an acidic solution to 1574 cm-1 of -NH2 in a basic solution were
observed.46
2.3 Photocatalytic H2 production of APA
The photosynthetic H2 evolution experiments of APA were carried out in an aqueous solution in
the presence of sodium ascorbate (NaHA) as an electron donor. Typically, a standard sample
containing PEI-Co (0.10 mg mL-1), CNPs (0.14 mg mL-1), and NaHA (0.02 M) in 5.00 mL
water (pH 8.4) was irradiated under a blue LED lamp (λmax = 450 nm) for 10 hours. A standard
sample produced 5.54 μmol H2 with a TON of 131 based on the cobalt sites (Table 1). No H2
was detected in the systems that were lacking PEI-Co, CNPs, NaHA, or light irradiation,
indicating that all these components are necessary for H2 production (Table 1). If PEI was used
to replace PEI-Co, the system produced only a trace of H2 under the same conditions (Table 1).
This result demonstrates that the cobalt complex in PEI-Co functions as a catalytic centre for H2
production.
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Table 1. Control experiments of photocatalytic H2 production[a]
H2 / μmol
H2 / μmol g -1 h-1
TON (H2)
Entry
Photosensitizer Catalyst
Electron donor
total amount
based on APA mass
based on cobalt
1[b]
2[b]
3[b]
4[c]
5[d]
6[e]
7[f]
8[g]
9[h]
CNPs
CNPs
CNPs
CNPs
/
PEI-Co
PEI-Co*
PEI-Co**
PEI-Co
PEI-Co
/
NaHA
NaHA
NaHA
NaHA
NaHA
NaHA
/
5.54
1.78
17.92
0
461.7
148.3
1493.5
0
131
124
68
0
0
0
0
CNPs
CNPs
CNPs
CNPs
0
0
0
PEI-Co
PEI
0
0
0
NaHA
NaHA
Trace
0.11
Trace
9.2
Trace
2.6
C1
[a] all samples were irradiated for 10 hours; [b] samples concentrations: [CNPs] = 0.14 mg
mL-1, [PEI-Co] = 0.10 mg mL-1 ([Co] = 8.46×10-6 M), [PEI-Co*] = 0.10 mg mL-1 ([Co] =
2.88×10-6 M), [PEI-Co**] = 0.10 mg mL-1 ([Co] = 5.24×10-5 M), [NaHA] = 0.02 M, total
volume = 5.00 mL; [c] in dark; [d] in the absence of CNPs; [e] in the absence of PEI-Co; [f] in
the absence of NaHA; [g] [PEI] = 0.10 mg mL-1; [h] [C1] = 8.46×10-6 M.
However, if complex C1 was used to replace PEI-Co to compose a non-assembled system under
the same conditions, the TON of H2 production was only 2.6 (Table 1). The APA system
improves H2 production by over 50 times compared with that of a non-assembled system.
The H2 production activity of the APA system is highly dependent on the concentrations of
CNPs and PEI-Co. As shown in Fig. 3a, a decrease in the CNPs concentration from 0.14 mg
mL-1 (standard sample) to 0.035 mg mL-1 decreased the TON of H2 production from 131 to only
7. An increase in the CNPs concentration to 0.28 and 0.56 mg mL-1 also resulted in a decrease in
the TON to 35 and 16, respectively, due to the light filter effect at a higher CNPs concentration.
A decrease in the PEI-Co concentration from 0.10 mg mL-1 in the standard sample to
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Figure 3. The photocatalytic H2 production by APA and reference systems in aqueous solutions,
pH 8.4, [NaHA] = 0.02 M, total volume = 5.00 mL, initial pH 8.4, irradiation time = 10 hours,
light source was a blue LED lamp (λmax = 450 nm). (a) [PEI-Co] = 0.10 mg mL-1, [CNPs] =
0.035-0.56 mg mL-1. (b) [PEI-Co] = 0.025-0.40 mg mL-1, [CNPs] = 0.14 mg mL-1. [c] [CNPs] =
0.14 mg mL-1, [PEI-Co] = 0.10 mg mL-1 ([Co] = 8.46×10-6 M), [PEI-Co*] = 0.10 mg mL-1
([Co] = 2.88×10-6 M), [PEI-Co**] = 0.10 mg mL-1 ([Co] = 5.24×10-5 M). [d] [PEI-Co] = 0.10
mg mL-1, [C1] = 8.46 × 10-6 M, [CNPs] = 0.14 mg mL-1, at different initial pH. (e) [CNPs] =
0.14 mg mL-1, [PEI-Co] = 0.10 mg mL-1 ([Co] = 8.46×10-6 M), [NaHA] = 0.02 M.
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0.025 mg mL-1 (Fig. 3b) resulted in a decrease in the TON to 60 because of low concentration of
the catalytic centres under these conditions. Interestingly, an increase in the PEI-Co
concentration from 0.10 mg mL-1 to 0.20 mg mL-1 and 0.4 mg mL-1 resulted in a decrease in the
TON to 16 and 2 (Fig. 3b), respectively. This decrease is due to the formation of additional APA
assemblies at a higher PEI-Co concentration; the local concentration of CNPs in each APA
assembly decreases, thus leading to a decrease in TON. As shown in Fig. 3c, by decreasing the
cobalt complex loading amount in PEI from 5.24 × 10-5 M in PEI-Co** to 2.88 × 10-6 M in PEI-
Co*, the H2 production amount decreased from 17.92 μmol to 1.78 μmol after 10 hours of
irradiation (Table 1), indicating the cobalt complexes function as a real catalytic site for protons
reduction. However, the maximum TON based on cobalt sites was obtained by a moderate cobalt
loading sample PEI-Co (Table 1), indicating that PEI-Co has the highest utilization efficiency
of the cobalt sites under the conditions employed.
The APA system had H2 production activity in a broad pH range from pH 4.0 to pH 10.1 (Fig.
3d); the highest TON of 131 was obtained at pH 8.4. However, the non-assembled system,
physical mixture of complex C1 and CNPs, produced H2 only in acidic solutions with pH 2.2-5.5.
The highest TON was 40 at pH 4.0. The amino groups of the PEI framework improve local
protons concentration in the vicinity of the cobalt catalytic centres and function as proton sponge
binding protons in water within a broad pH range and subsequently delivering protons to the
catalytic cobalt centre during photocatalysis.34-36
Based on these results, a long-time photocatalysis test of the APA system by using PEI-Co was
performed under the optimized conditions. The system maintained the H2 production activity
within the first 24 hours (Fig. 3e). After 34 hours of irradiation, a total of 8.36 μmol H2 with a
TON of 198 was obtained. The highest H2 production rate based on CNPs@PEI-Co mass is 538
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μmol g-1 h-1. To the best of our knowledge, the APA system is the most efficient photocatalytic
H2 production system based on carbon dots/nanoparticles as a photosensitizer and cobalt
complex as a catalyst reported so far. To reveal the cause of inactivation, we added fresh CNPs
or PEI-Co into the inactivated samples for another 10 hours of irradiation. However, both
samples produced only small amount of H2 (SI, Table S1). Confocal observation of the solution
from the inactivated sample showed that the rod-like structures of PEI-Co still exist, however,
no obvious emissive assemblies can be observed from green confocal channel (SI, Fig. S5). The
comparison of the UV-vis absorption spectra of the photocatalytic sample before and after 34
hours of irradiation demonstrated that the characteristic absorption of CNPs disappeared (SI, Fig.
S7). These results indicate that both CNPs and cobalt complexes in PEI-Co decomposed during
long-time photocatalysis.
2.4 Photoinduced electron transfer
To gain an insight into the photocatalytic mechanism of the APA system, the photoinduced
electron transfer (PET) process was investigated. Excitation of an aqueous solution of CNPs at
400 nm results in a maximal emission at 548 nm (Fig. 4a) with an average lifetime of 1.11 ns (SI,
Fig. S8). This emission can be efficiently quenched by PEI-Co (Fig. 4a) with a quenching
constant of 3.69 × 1013 M-1 s-1 (SI, Fig. S9). However, the quenching constant is decreased to
1.32 × 1013 M-1 s-1 by using C1 to quench the aqueous solution of CNPs in the presence of PEI
under the same conditions (SI, Fig. S10). Due to a negligible spectroscopic overlap of the
absorption of PEI-Co with emission of CNPs (Fig. 4a), photoinduced energy transfer between
these two components can be excluded. We therefore attributed this quenching to PET from
excited CNPs to the cobalt complexes in PEI-Co. The free-energy change (ΔG0) of the PET
process was estimated. The oxidation potential (Eox) of CNPs and the reduction potential (Ered) of
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Figure 4. (a) UV-Vis absorption of PEI-Co (0.10 mg mL-1) in aqueous solution, emission
quenching spectra of CNPs (0.14 mg mL-1) by PEI-Co (2.12 × 10-6 - 1.27 × 10-5 M) in aqueous
solution at 400 nm excitation wavelength. (b) Transient photocurrent response to on-off
illumination, (c) linear sweep voltammetry, and (d) electrochemical impedance spectroscopy
curves of CNPs and CNPs@PEI-Co modified electrodes in Na2SO4 (0.1 M) aqueous solution
under a Xe lamp (λ > 400 nm) irradiation.
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C1 in aqueous solution is 0.85 V and -1.47 V, respectively (all potentials discussed here are vs.
SCE), as measured by cyclic voltammograms (SI, Fig. S12 and S13). The excited state energy
(E00) of CNPs is 2.49 eV. The free energy change (ΔG0) was determined as -0.17 eV, indicating
that PET from CNPs to the cobalt complexes in PEI-Co in the CNPs@PEI-Co assemblies is
exothermic (Scheme 2).
Photoelectrochemical measurements of CNPs and CNPs@PEI-Co were performed. As shown in
Fig. 4b, the CNPs modified electrode exhibits a transient photocurrent of 0.31 μA cm-2 upon
irradiation by a Xe lamp (λ > 420 nm). The photocurrent increased to 0.67 μA cm-2 after loading
PEI-Co onto CNPs electrode. These photocurrents of CNPs and CNPs@PEI-Co were roughly
stable in the test period. The comparison of linear sweep voltammetry (LSV) and
electrochemical impedance spectroscopy (EIS) curves for CNPs and CNPs@PEI-Co modified
electrodes shows that CNPs@PEI-Co have higher photocurrent density in LSV (Fig. 4c) and
smaller radius of the semicircle in EIS (Fig. 4d) compared with those of CNPs. These results
further demonstrate that PET from excited CNPs to PEI-Co takes place and leads more efficient
catalysis in CNPs@PEI-Co.47, 48
2.5 Mechanism
Based on these results, a mechanism of H2 production by the CNPs@PEI-Co assemblies can be
proposed (Scheme 2). Under visible light irradiation, excited CNPs delivers a photoexcited
electron to the cobalt centre in PEI-Co to generate active CoI species; subsequently, this CoI
species binds a proton to undergo a catalytic H2 production cycle (SI, Fig. S12).49-51 The oxidized
CNPs can be reduced to the initial state by ascorbate (SI, Fig. S11). The specific structure of
CNPs@PEI-Co envelops the active cobalt centre and CNPs into an assembly of several
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Scheme 2. Scheme of photocatalytic H2 production process in the APA system and energy
diagram of the photoinduced electron transfer.
micrometers in size formed by PEI; apparently, the assembly is beneficial for H2 production for
three reasons. First, the assembly positions light harvester CNPs and catalytic cobalt centre
adjacent to each other and locate at the surface of the assemblies, thus facilitating PET efficiency
and light absorption. Second, abundant amino groups in PEI may be present in the vicinity of the
cobalt centre in the CNPs@PEI-Co assemblies; hence, these amino groups function as protons
relays to facilitate protonation at the catalytic metal centre during the course of hydrogen
production; a similar strategy, having a pendant amino group as a proton relay at the secondary-
coordination sphere of the metal centre, is observed in natural [FeFe]-hydrogenases.52, 53 Third,
the residual abundant amino groups in PEI framework act as a proton sponge to enhance local
proton concentration in assemblies to enable their function within a wide pH range. All these
characteristics of CNPs@PEI-Co assemblies are reminiscent of photosynthetic bacteria in nature,
in which active catalytic centres of the metal clusters, light-harvesting pigments and protons
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relay are precisely assembled in the proteins and stay at cell membrane for efficient
photosynthesis.
3. Conclusions
In conclusion, a kind of biomimetic artificial photosynthetic assemblies of a hollow-rod structure
was successfully constructed by using self-assembly of synthetic building blocks containing only
inexpensive elements.25-29 The APA features morphological and structural characteristics like
those of NPB and perform photosynthetic H2 evolution under visible light irradiation. Due to the
special structure and multiple functionalities of the CNPs@PEI-Co assemblies, the APA system
demonstrates enhanced photocatalytic H2 production within a broad pH range in aqueous
solution. The performance of the artificial photosynthetic systems can be improved by
mimicking the structure and function of natural biological systems.
Experimental section
Chemicals and Materials. All chemicals, including 2, 6-bis(chloromethyl)pyridine, 2, 2-
dipicolylamine (DPA), N-Bromosuccinimide (NBS), Benzoyl peroxide (BPO), potassium
carbonate, benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate (PyBOP) and
cobalt chloride hexahydrate (CoCl2·6H2O) were purchased from commercial suppliers
(Sinopharm chemical reagent co., LTD, Adamas, and Sigma-Aldrich) and used without further
purification. N2 (99.999%), CH4 (99.99%) were purchased from commercial supplier
(Huaerwen). All solvents of analytical grade were purchased from commercial suppliers and
used without further purification. The ligand L1-L3,54, 55 complex C1,56 PEI-Co,31 and CNPs40
were synthesized according to literatures’ methods.
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Synthesis of L1. Methyl 6-methylnicotinic acid (200 mg, 1.32 mmol), NBS (259 mg, 1.46
mmol), and BPO (32 mg, 0.13 mmol) were added to 20 mL of CCl4. The mixture was stirred and
heated to reflux. After the completion of the reaction, the solution was filtered to remove the
solid. The filtrate was washed by saturated NaHCO3 aqueous solution and brine. The organic
layer was dried over MgSO4 and concentrated by rotary evaporation to give a solid. The solid
was purified by column chromatography and eluting with CH2Cl2. The product L1 (yield:
1
41.8%) as a white solid was obtained by concentration under vacuum. H NMR (400 MHz,
CDCl3) δ 9.17 (s, 1H), 8.33 (dd, J = 8.1, 2.2 Hz, 1H), 7.56 (d, J = 8.1 Hz, 1H), 4.60 (s, 2H), 3.97
(s, 3H).
Synthesis of L2. L1 (100 mg, 0.43 mmol), DPA (105 mg, 0.52 mmol), and K2CO3 (120 mg,
0.87 mmol) were added in 10 mL of CH3CN at room temperature. The reaction mixture was
stirred at 50 ℃ for 4h. After the completion of the reaction, the mixture was concentrated and
purified by column chromatography and eluting with CH2Cl2/CH3OH mixed solution. The
product L2 (yield: 84.3%) as an oil was obtained by concentration under vacuum. 1H NMR (400
MHz, CDCl3) δ 9.16 - 9.07 (m, 1H), 8.54 (ddd, J = 4.9, 1.8, 0.9 Hz, 2H), 8.25 (dd, J = 8.2, 2.2
Hz, 1H), 7.73 - 7.61 (m, 1H), 7.58 - 7.50 (m, 2H), 7.15 (ddd, J = 7.5, 4.9, 1.2 Hz, 2H), 3.95 (s,
2H), 3.93 (s, 3H), 3.89 (s, 4H).
Synthesis of L3. L2 (100 mg, 0.29 mmol) and NaOH aqueous solution (0.1 M, 20 mL) were
dissolved in 20 mL of THF and stirred for 3h at 0 ℃. After the completion of the reaction, the
THF was removed by rotary evaporation. The HCl (3 M) was added into the residual solution to
adjust the pH to 6-7 and the water was removed by rotary evaporation. The solid crude product
was dissolved in CH3OH and dried over MgSO4. The solution was filtered and the filtrate was
1
concentrated under vacuum to obtained the product L3 (yield: 95%) as a pale yellow solid. H
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NMR (400 MHz, Methanol-d4) δ 8.97 (d, J = 2.0 Hz, 1H), 8.44 (dd, J = 5.0, 1.7 Hz, 2H), 8.21
(dd, J = 8.1, 2.2 Hz, 1H), 7.74 (td, J = 7.7, 1.8 Hz, 2H), 7.54 (dd, J = 7.9, 4.9 Hz, 3H), 7.23 (ddd,
J = 7.6, 5.0, 1.2 Hz, 2H), 3.83 (d, J = 9.0 Hz, 6H). 13C NMR (101 MHz, MeOD) δ 171.64,
161.72, 159.63, 151.15, 149.90, 139.14, 138.69, 132.23, 124.96, 124.03, 123.90, 61.18, 60.96.
MS (ESI, m/z): Calcd. For C19H18N4O2 [M+Na]+ : 357.1327; found, 357.1340.
Synthesis of C1. L3 (100 mg, 0.30 mmol) was dissolved in 3 mL of CH3OH to obtain a brown
solution. CoCl2·6H2O (64 mg, 0.27 mmol) was added into the solution at room temperature. The
color of the solution changed from brown to green. The mixture was stirred at room temperature
for 1 h. Diethyl ether (20 mL) was added into the solution to give green precipitates. The product
C1 was obtained as a green solid (yield: 90%) after filtration, washing with water and ether, and
drying under vacuum. MS (ESI, m/z): Calcd. For C19H18Cl2CoN4O2 [M-Cl]+ : 428.0450; found,
428.0422.
Synthesis of PEI-Co. C1 (40 mg) and PyBOP (53 mg) were dissolved in 15 mL of DMF and
stirred at room temperature for 30 min. PEI (200 mg, Mw = 25 k) in 3 mL of DMF was added
drop by drop into the solution. The mixture solution was stirred at room temperature for 48 h.
The solution was filtrated to remove the with flocculent. The filtrate was dialyzed (3000 D) in
water and then freeze-dried to obtain the PEI-Co ([Co] = 8.46 × 10-6 mmol mg-1) as a brown
solid. Two reference samples, PEI-Co* ([Co] = 2.88 × 10-6 mmol mg-1) and PEI-Co** ([Co] =
5.24 × 10-5 mmol mg-1), of different cobalt loading amount were also prepared by the same
procedure.
Synthesis of CNPs. The CNPs was synthesized by hydrothermal treatment of poly-3-
thiopheneacetic acid in NaOH aqueous solution (100 mM, 40 mL) in a hydrothermal reactor
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(Shanghai LABE Instrument Co., Ltd.) at 210 ℃ for 6 h. After cooling to room temperature, the
CNPs were collected and purified by sequential filtration, centrifugation, and dialysis.
Photocatalytic experiments. In a typical photocatalytic H2 evolution experiment, the stock
aqueous solution of CNPs, PEI-Co, and sodium ascorbate (NaHA) were added in a glass tube to
obtain a mixture solution with certain components’ concentration. The sealed tube was bubbled
by N2 for 20 mins under dark. The tubes were set in a photoreactor with blue LED lamp (λmax
=
450 nm) and irradiated at room temperature for certain time. The gaseous product was extracted
and analyzed by a gas chromatography (Shanghai Tianmei, GC 7900) equipped with a thermal
conductive detector (TCD) and flame ionization detector (FID) having high-purity Ar carrier gas.
The CH4 was used as an internal standard.
Electrochemical and photoelectrochemical measurements. Cyclic voltammetries were
performed in a three-electrode system (0.2 M Na2SO4 aqueous solution) of using glassy carbon
electrode as a working electrode, saturated calomel electrode (SCE) as a reference electrode, and
a platinum wire as a counter electrode. Photoelectrochemical measurements were conducted in a
three-electrode cell system by using 0.1 M Na2SO4 aqueous solution as electrolyte. An FTO
glass of loading CNPs or CNPs@PEI-Co was used as a photoelectrode, a Pt wire and a SCE
were used as counter electrode and reference electrode, respectively. The photoelectrode was
prepared by spin-coating CNPs or CNPs@PEI-Co slurry on an FTO glass (d = 4 mm) and then
drying at 40 °C. The light source was a 300 W xenon lamp with a light filter (λ > 420 nm).
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ACKNOWLEDGMENT
We are grateful for financial support from the National Natural Science Foundation of China
(21871102) and the Fundamental Research Funds for the Central Universities
(2019kfyRCPY101). We thank the Analytical and Testing Center of HUST for measurements.
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90x39mm (300 x 300 DPI)