Cadmium sulfide net framework nanoparticles for photo-catalyzed cell redox

Article abstract of DOI:10.1039/d0ra08235j

A strategy for synthesizing cadmium sulfide net framework (CdS-NF) nanoparticles was developed in a water-based system under mild reaction conditions. The CdS-NFs have not only the excellent photocatalytic properties of CdS, but also the large surface area and diverse porous structures of a metal-organic framework. An Escherichia coli-CdS-NF hybrid system was constructed using NADH regeneration to promote the conversion of trimethylpyruvate acid to l-tert-leucine. The E. coli-CdS-NF system showed higher NAD+ recycling efficiency and substrate conversion rate than CdS QDs under visible light illumination. This work demonstrates a novel method for developing a brilliant coenzyme recycling photocatalyst in bio-redox reactions.

Full text of DOI:10.1039/d0ra08235j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Cadmium sulfide net framework nanoparticles for
photo-catalyzed cell redox†
Cite this: RSC Adv., 2020, 10, 37820
Zhaoyu Chang,‡ Jian Zhang,‡ Wanyuan Dong, Xiangqi Meng, Hualei Wang,
*
Dongzhi Wei
and Yuhong Ren
A strategy for synthesizing cadmium sulfide net framework (CdS-NF) nanoparticles was developed in
a water-based system under mild reaction conditions. The CdS-NFs have not only the excellent
photocatalytic properties of CdS, but also the large surface area and diverse porous structures of
a metal–organic framework. An Escherichia coli–CdS-NF hybrid system was constructed using NADH
regeneration to promote the conversion of trimethylpyruvate acid to L-tert-leucine. The E. coli–CdS-NF
system showed higher NAD+ recycling efficiency and substrate conversion rate than CdS QDs under
visible light illumination. This work demonstrates a novel method for developing a brilliant coenzyme
recycling photocatalyst in bio-redox reactions.
Received 22nd July 2020
Accepted 7th October 2020
DOI: 10.1039/d0ra08235j
rsc.li/rsc-advances
reported. Octadecene was used as a non-coordinating solvent
for the synthesis of high-quality CdS nanocrystals.16 CdS
1. Introduction
quantum dots (CdS QDs) were synthesized at high temperature
to regenerate NADH.17
Oxidoreductase plays an important role in the redox reaction. It
catalyzes the reaction with cofactors for hydrogen sources and
energy such as nicotinamide adenine dinucleotide (NADH) and
its phosphorylated form (NADPH). However, cofactors are
expensive and difficult to use in large-scale production.1,2 Based
on this situation, many researchers have focused on the
regeneration of cofactors. The regeneration methods have been
divided into six types.3 Among them, photocatalytic regenera-
tion has attracted increasing interest from researchers because
the amount of solar energy is endless and half of the total
sunlight is in the visible range.4–6
MOFs, also known as porous coordination polymers, are
composed of metal ions or clusters as well as organic ligands
through coordination bonds. MOFs have many special charac-
teristics such as diverse porous structures, large surface areas
and excellent mechanical stability.18 Conventional nanocrystals
cannot be ne-tuned and crystallized, thus exhibit inhomoge-
neity and long-range order from the atomic to micron scale
regime that result in low protein loading efficiency and unsta-
bility.19 Compared with traditional semiconductors, MOFs are
considered promising candidates for cell catalysis applications.
The zeolitic imidazolate framework-8 (ZIF-8) is a representative
MOF formed by coordination between Zn2+ and 2-methyl-
imidazole (2-MI).20
In this study, CdS was selected to transfer electrons to
coenzyme NAD+ to regenerate NADH. An efficient coenzyme
self-circulation reaction was formed to promote the progression
of the biocatalytic reaction. To improve the efficiency of the
semiconductor-catalyzed redox reactions, we synthesized CdS
net frameworks (CdS-NFs) via MOFs. Herein, Cd2+ was used to
replace Zn2+ in composite MOF-CdCl2 for the rst time and
sodium sulde was then added to produce a CdS net framework
structure. CdS is dispersed in the net framework, which is
benecial to transfer electrons to the cell redox reaction.
NAD+ requires one proton and two electrons to regenerate
NADH. Some semiconductors absorb solar energy to transfer
protons and electrons with small bandgaps.7 Three kinds of
electron donors were oen selected including triethanolamine8
(TEOA), 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid9
(HEPES), and ascorbic acid (VC). Cadmium sulphide (CdS) has
attracted the attention of researchers not only for its
outstanding semiconductor optical and electrical properties but
also for its suitable spectral absorption range.10,11 The bandgap
of CdS is 2.4 eV, and it can absorb visible light when semi-
conductor has a narrow band gap (Eg < 3.0 eV).12 CdS nano-
particles have been used in many regions such as photocatalytic
hydrogen production, solar batteries, and redox reactions.13–15
In recent years, many different forms of CdS have been
State Key Laboratory of Bioreactor Engineering, New World Institute of Biotechnology,
East China University of Science and Technology, Shanghai 200237, China. E-mail:
yhren@ecust.edu.cn; Fax: +86 21 6425 0068; Tel: +86 21 6425 2163
† Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S3. See DOI: 10.1039/d0ra08235j
2. Experimental section
2.1 Materials and reagents
The plasmid pET28a-LDH was previously constructed in our
‡ These authors contributed equally to this work.
laboratory. Cadmium chloride, sodium sulde and ammonium
37820 | RSC Adv., 2020, 10, 37820–37825
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
chloride were purchased from Yonghua Chemical Co., Ltd.
(Shanghai, China). Trimethylpyruvate acid, 2-methylimidazole,
and polyvinylpyrrolidone were purchased from Sigma-Aldrich
(Shanghai, China). All other chemicals were purchased from
Adamas-beta (Shanghai, China).
2.5 Aggregation of cell-CdS
Three different CdS were weighed in a beaker and dissolved
with 2 mL pure water to the concentration of 3 g Lꢂ1 and then
sonicated for 15 min. The lyophilized resting cells were added to
the beaker and the ratio of CdS to cells was set to 3 : 1. The
mixed solution was then agitated for 1 hour at 4 ꢁC and the
aggregates were collected by centrifugation at 7500 ꢀ g for
10 min. The cell-CdS aggregates were washed with pure water
for three times.
2.2 Synthesis method of CdS-NFs
The 10 mM cadmium chloride (CdCl2$2.5H2O) dissolved in 50%
ethanol was added to a round bottom ask. 2-Methylimidazole
(2-MI) and polyvinylpyrrolidone polymer (PVP, Mw ¼ 40 000)21
were dissolved in 50% ethanol and added into the ask with
a constant pressure separatory funnel to control the dropping
rate at 5–10 drops per second. Agitation and sonication were
used to accelerate the reaction. In this case, the Cd2+/2-MI ratio
and the concentration of PVP were 1 : 4 and 2 wt%. Aer 20 min
of reaction, the reaction solution was centrifuged (8000 ꢀ g, 10
min) and the precipitation was washed three times with pure
ethanol and ultra-pure water. The precipitation was redissolved
with 50% ethanol and added to a round bottom ask. The
10 mM Na2S was added dropwise using a constant pressure
separatory funnel. The bright yellow precipitation was observed
obviously aer reaction for 20 min. The reaction mixture was
centrifuged (6000 ꢀ g, 10 min). The precipitation was washed
three times with pure ethanol and pure water, and then freeze-
dried for subsequent experiment. All reactions were performed
at room temperature. The Cd2+/2-MI ratio could be adjusted to
2.6 Characterizations
To characterize and optimize the synthesized CdS nano-
particles, multi-angle laser light scattering (MALS) was carried
out on a Wyatt GPC/SEC (America). The samples were diluted to
1 g Lꢂ1. The XRD experiments were performed on a Rotating
Anode X-ray Powder Diffractometer with model i18KW/D/
max2550VB/PC operating at 40 kV and 100 mA. The patterns
were collected over a 2q range from 5ꢁ to 75ꢁ with a scanning
speed of 12ꢁ minꢂ1. Nitrogen adsorption and desorption
isotherms were performed on an ASAP 2020 analyzer (Micro-
meritics, USA) at 77 K aer samples degassed under a vaccum of
623 K for 6 h. The total pore volumes were determined under
the condition of p/p0 ¼ 0.99. The Brunauer–Emmett–Teller
(BET) method was used to determine the surface areas and the
pore volumes.
Three kinds of CdS nanoparticles were characterized by eld-
emission scanning electron microscopy (FE-SEM). Nano-
particles were dissolved in ethanol and dispersed by sonication
for 10 min, and then added (10 mL) to a silicon wafer and dried
at room temperature. The eld-emission scanning electron
microscopy images were observed on Gemini 500 SEM (Ger-
many). In addition, the crystal lattice of CdS in CdS-NF was
characterized by high-resolution transmission electron for JEM
2100 (Japan). To characterize the cell-CdS aggregates, trans-
mission electron microscopy (TEM) was used to observe the
binding of cells to nanoparticles. The aggregated samples were
resuspended with pure water, and 10 mL suspension was added
to a copper mesh dried at room temperature. The images were
characterized by JEM 1400 (Japan).
1 : 2, 1 : 4 and 1 : 8. The CdS-NF precipitation formed with Cd2+
/
2-MI ¼ 1 : 4 was washed with pure ethanol and pure water three
times. Aer centrifugation, the supernatant was found to be
bright yellow when the pellet was washed with water. The
supernatant was placed in a 50 mL centrifuge tube at room
temperature for a period of time, and then the CdS-NF solution
was aggregated to form the precipitation.
2.3 Synthesis method of CdS quantum dots
The CdS QDs were synthesized in aqueous solution. The
CdCl2$2.5H2O (10 mM) and thiourea (10 mM) were added to
ꢁ
a 250 mL round bottom-ask, heated at 150 C and stirred for
3 min to make them evenly mixed. Na2S$9H2O (10 mM) was
added to the reaction solution rapidly, and the mixing system
continued with stirring and cooling at 100 ꢁC at the same time.
Aer reacting for 10 min, the solution was cooled to room
temperature. The solution was centrifuged (7500 ꢀ g, 10 min)
and the precipitation washed three times with pure ethanol and
pure water.
2.7 Optimization of photocatalytic conditions and detection
of trimethylpyruvate acid by HPLC
A 3 mL transparent vial was subjected to photocatalysis in water
ꢁ
bath at 30 C under 460 nm light. The photocatalytic reaction
system contained 200 mM HEPES (pH ¼ 8.0), 200 mM ammo-
nium chloride, 1 g Lꢂ1 cells, 10 mM trimethylpyruvate acid,
electron donor, and photocataꢁlyst under 460 nm LED light.
2.4 Escherichia coli expression
Leucine dehydrogenase from Labrenzia aggregate was con- Aer reacting for 72 h in a 30 C water bath, 300 mL aliquots
structed in pET-28a and then transformed into Escherichia coli were taken every 12 h and centrifuged at 13 000 ꢀ g for 7 min.
BL21 (DE3). The strain was cultured in 200 mL Luria broth (LB) The substrate sample liquid analysis method has been
at 37 C. The 1 M isopropyl b-D-1-thiogalactopyranoside (IPTG) reported.22
ꢁ
was added to ask when the optical density at 600 nm (OD600
)
The electron donor sacrices electrons to the photocatalyst
reached 0.6, and the protein expression was induced at 18 ꢁC for to avoid recombining electrons and holes. The effect of different
20 h. The cell cultures were collected by centrifugation at 7500 electron donors on the photocatalytic reactions was determined
ꢀ g for 10 min, and then the cell pellet was freeze-dried.
by three different electron donors, including 200 mM HEPES,
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37820–37825 | 37821

View Article Online
RSC Advances
Paper
150 mM TEOA, and 200 mM VC. The reaction was performed for at 7500 ꢀ g for 10 min. The precipitation was washed with pure
72 h under 460 nm LED illumination. ethanol and ultra-pure water to obtain MOF-CdCl2 (Fig. 1A).
The conversion of trimethylpyruvate acid to L-tert-leucine Sodium sulde was added to the reaction system aer dissolv-
requires reducing force provided by NADH cofactor. The ing MOF-CdCl2 with ethanol. As the reaction proceeded, bright
ꢁ
substrate was catalysed in water bath at 30 C for 72 h and the yellow precipitation was appeared in the reaction vessel and
conversion efficiency was measured by HPLC to determine the identied as CdS-NFs (Fig. 1A).
effect of exogenous 0.5 mM NADH and 1 g Lꢂ1 cells containing
The synthesis conditions of CdS-NFs including the reaction
cofactors on the photocatalytic reaction. The effect of photo- solution and the ratio of Cd2+/2-MI were optimized in this study.
catalyst concentration on the photocatalytic reaction was The size of CdS-NFs was characterized by Wyatt GPC/SEC-MALS.
determined by setting CdS-NF concentration gradients (2, 5, CdS-NFs synthesized in ethanol had small particles (ꢃ10 nm;
and 10 mM) to analyse the substrate conversion rate. The Fig. S1†). For comparison, the molar ratio of Cd2+/2-MI in the
inuence of LED illumination on the photocatalytic reaction ethanol system was changed to obtain different sizes of prod-
was determined by 460 nm LED light illuminate reaction for ucts. Cd2+/2-MI ¼ 1 : 2 and 1 : 4 were set in this experiment. The
72 h, and dark condition was used as a control group. The effect yield of the precipitation at Cd2+/2-MI ¼ 1 : 4 (23.4%) was
of light intensity on photocatalytic reaction was determined by greater than Cd2+/2-MI ¼ 1 : 2 (18.3%). However, the particle
setting the light intensity gradients (1.86, 2.29, and 3.77 mW distribution was similar in both systems, ranging from 1 nm to
cmꢂ2) to measure the substrate conversion rate.
100 nm (Fig. S2†). We further compared the particle size of
MOF-CdCl2 and CdS-NFs. The particle size of CdS-NFs was
larger than MOF-CdCl2 when sodium sulde was added into the
system (Fig. 1B and C).
The CdS-NF precipitation was washed with ultrapure water
and then centrifuged. The supernatant was bright yellow, which
indicated the formation of CdS-NF nanoparticles. Aer keeping
at room temperature for a period of time, the bright yellow
supernatant was aggregated to form precipitation.
To compare the photocatalysis efficiency of CdS-NFs with
traditional CdS, CdS quantum dots (QDs) was introduced in this
study. CdS QDs were synthesized under high temperature
conditions, resulting in the formation of small particles of
about 5 nm, but they were difficult to dissolve in water (Fig. 1D).
3. Results and discussion
3.1 CdS nanoparticles assay
In this reaction system, cadmium chloride was dissolved in
ethanol which used as the triggered solvent; 2-methylimidazole
(2-MI) and polyvinylpyrrolidone polymer (PVP, Mw ¼ 40 000)
were served as stabilizers and added to the mixed solution with
agitation and ultra-sonication. Aer 20 min of mixing, white
precipitation was appeared and then collected by centrifugation
3.2 X-ray diffraction
X-ray diffraction analysis of the CdS samples obtained from
different synthesis methods revealed that CdS QDs and CdS-NF
nanoparticles were corresponding with cubic lattice. The peak
can be classied as the 111, 220, and 311 (PDF, le no. 10-0454).
The crystallite sizes of CdS QDs and CdS-NFs were estimated to
be 2.85 and 5.61 nm, according to the Scherrer equation
(Fig. 2).23
The pattern of CdS samples showed that synthesis temper-
ature had a great inuence on the formation of nanocrystals.
The diffraction peaks would be shaped and the crystallization
would be better as the synthesis temperature rose. The CdS-NF
nanoparticles were synthesized at room temperature and the
peak was broadened. In addition, an amorphous state was
formed with the addition of organic materials and polymers,
which resulted in weaker peak intensity of CdS-NFs.
Fig. 1 (A) Precipitation of synthetic MOF-CdCl2 and CdS-NFs. (B)–(D)
CdS nanoparticles were characterized by Wyatt GPC/SEC-MALS.
Table 1 Porous characteristic of CdS samples
BET surface
area (m2 gꢂ1
Pore volume
Pore size
(nm)
Samples
)
(m3 gꢂ1
)
CdS QDs
CdS-NFs
158.90
142.79
0.27
0.41
7.3
20
Fig.
2 X-ray diffraction spectrum of CdS QDs and CdS-NF
nanoparticles.
37822 | RSC Adv., 2020, 10, 37820–37825
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
microscope (HRTEM) (Fig. 4A, B and S3†). The images showed
that the supernatant of CdS-NF formed a network and porous
structures. CdS was combined with the network structure, and
the particles size were about 5 nm (Fig. 1A and B). The FE-SEM
image indicated that the precipitation was composed of two
nanomaterials: one was CdS-NFs that formed the net frame-
work, and the other was a shuttle structure of about 200 nm.
The sediment was washed by ultra-pure water until CdS could
not be detected in the supernatant. The precipitation was then
characterized by FE-SEM and the results showed that the shuttle
structure could not be found. There was only the network
structure detected in precipitation (data not shown), which
indicated that the shuttle-shaped materials were aggregates of
CdS-NFs (CdS-NFs-A). Only shuttle-like aggregates without the
CdS network structure were obtained when the ratio of Cd2+/2-
MI ¼ 1 : 8 (Fig. 4C). The size of the CdS-NFs-A particles was
more than 500 nm. The morphology of CdS QDs was charac-
terized by FE-SEM in Fig. 4D. The size of the CdS QD particles
was less than 5 nm. However, CdS QDs were aggregated easily
and difficult to disperse.
Next, to demonstrate the photocatalytic performance of CdS-
NF semiconductors, leucine dehydrogenase (LDH) from Lab-
renzia aggregate was selected to combine with CdS-NFs through
electrostatic interactions. Leucine dehydrogenase was con-
structed in Escherichia coli BL21 (DE3) and a cell-CdS hybrid
system was developed. The hybrid system makes use of the CdS
to transfer proton and electron to regenerate NADH in the
conversion of trimethylpyruvate acid to L-tert-leucine.
The morphology of the cell-CdS-NF, cell-CdS-NFs-A and cell-
CdS QD aggregates was observed by TEM (Fig. 5). CdS-NFs were
bound to the cell via non-covalent interactions, and the
combined areas increased because of the net framework
structure (Fig. 5A). CdS-NFs and CdS-NFs-A were synthesized via
organic substances, and the cell membrane is composed of
lipid-soluble substances. We speculated that the organic matter
Fig. 3 N2 adsorption (solid symbols) and desorption (open symbols)
for CdS QDs and CdS-NFs. The inset image shows the mesopore size
distribution.
3.3 Nitrogen adsorption and desorption curves
Nitrogen adsorption and desorption isotherms in Fig. 3 showed
the textural characteristics of CdS QDs and CdS-NFs. The BET
surface area, pore volume, and pore size were presented in
Table 1. At very low particle pressure (p/p0 < 0.1), the nitrogen
physical adsorption of CdS QDs exhibited a steep absorption,
which conrmed the presence of micropores.24 CdS-NFs dis-
played a combination of type II isotherm and H4 hysteresis loop,
and were characterized by a relative pressure p/p0 of 0.43. This
type of hysteresis loop suggested that the CdS-NF nanoparticles
contain mesopores.25 CdS QDs nanoparticles are easy to
aggregate and lead to the existence of hysteresis loop, which can
be proved in the FE-SEM images of CdS QDs (Fig. 4D).
3.4 Electron microscope assay
The size and morphology of the precipitation and supernatant
were characterized by eld emission scanning electron micro-
scope (FE-SEM) and high-resolution transmission electron
Fig. 4 CdS-NF, CdS-NFs-A, and CdS QDs were characterized by FE-
SEM and TEM. (A) FE-SEM image of the supernatant of CdS-NF. Scale Fig. 5 TEM images showed the morphology of three nanoparticles
bar ¼ 100 nm; (B) TEM image of the supernatant of CdS-NF; (C) FE- combined with Escherichia coli. (A) The morphology of cell-CdS-NF
SEM image of the CdS-NFs-A formed with Cd2+/2-MI ¼ 1 : 8. Scale bar aggregates observed by TEM. (B) The morphology of cell-CdS QD
¼ 1 mm; (D) the morphology and size of CdS QDs particles were aggregates. Scale bar ¼ 0.2 mm. (C) An image of cell-CdS-NFs-A
observed by FE-SEM. Scale bar ¼ 20 nm.
aggregates. Scale bar ¼ 0.5 mm.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37820–37825 | 37823

View Article Online
RSC Advances
Paper
trimethylpyruvate acid reacted aer 72 h of illumination when
2 mM CdS-NFs were added to the hybrid system (Fig. 6C). To
improve the efficiency of the reaction, a longer reaction time was
required in this experiment. When the CdS-NF concentration
increased to 10 mM, the substrate consumption rate was as high
as 30% within 12 h, which is similar to the result of 5 mM CdS-NF
photocatalysis. However, the substrate conversion rate curve
gradually became smooth as the reaction progressed. We sus-
pected that high concentration of CdS-NFs would be toxic to cells.
Therefore, 5 mM CdS-NFs were selected for photocatalysis.
3.6 Effect of LED illumination, and light intensity in
photocatalysis reaction
The substrate conversion rate of CdS-NF photocatalysis was only
10% in the absence of 460 nm LED illumination, which is close
to the control group that without CdS-NFs. The trimethylpyr-
uvate acid conversion rate was as high as 95% at 72 h (Fig. 7A),
and the photocatalytic efficiency was increased by 9.5-fold
under the light. The efficiency of photocatalytic reactions is thus
clearly dependent on illumination. Three types of photocatalytic
reactions with different light intensities were studied. The
substrate conversion rate was 93% under an illumination of
2.29 mW cmꢂ2 and showed a 1.3-fold increase compared with
3.77 mW cmꢂ2 light intensity. These indicated that the illumi-
nation intensity also affects CdS-NF photocatalysis (Fig. 7B).
Therefore, the illumination intensity must be controlled in
photocatalysis because high intensity light will destabilize the
enzymes in the cell when the photocatalyst is present. An illu-
mination intensity of 2.29 mW cmꢂ2 was nally selected.
Fig. 6 Effect of electron donor, NADH, and photocatalyst concen-
tration in photocatalysis reaction. (A) Effects of three electron donors
on CdS photocatalysis; (B) the influence of NADH on photocatalysis;
and (C) CdS-NF concentration affects photocatalytic efficiency.
contained in CdS-NFs has a high affinity with the fat-soluble cell
membrane. The morphology of the cell-CdS QD aggregates
changed from rod-shaped to spherical, which is due to the self-
aggregation of CdS nanoparticles that bound to the cell surface
(Fig. 5B). This phenomenon was not observed in the cell-CdS-NF
and cell-CdS-NFs-A aggregates (Fig. 5C), which indicated that
the network acted as surfactant to avoid CdS aggregation.
3.5 Effect of electron donor, NADH, and photocatalyst
concentration on the efficiency of photocatalysis reaction
3.7 Efficiency of different CdS nanoparticles in
HEPES was used as the reaction buffer solution, and TEOA was
the electron donor (Fig. 6A). Ascorbic acid was not suitable as an
electron donor in this reaction system because the optimal pH
of LDH is 8.5. The substrate conversion efficiency of the coen-
zyme contained in the cell was as high as 91% aer 72 h of
irradiation, which was similar to the addition of NADH (Fig. 6B).
This result suggested that exogenous addition of 0.5 mM NADH
did not participate in intracellular photocatalysis; therefore, the
CdS-cell reaction system does not need exogenous NADH.
The inuence of CdS-NF concentration and illumination
intensity on photocatalysis was studied. Only 6.5 mM
photocatalytic reaction
Three kinds of CdS including CdS QDs, CdS-NFs-A and CdS-NFs
were added to the hybrid photocatalytic reaction respectively.
The results showed that the conversion rate of CdS-NF photo-
catalytic trimethylpyruvate acid was as high as 95% under
optimal reaction conditions, and the cofactor-recycling
Fig. 8 The conversion rate of trimethylpyruvate acid that photo-
catalyzed by three different kinds of CdS. The reaction system includes
200 mM HEPES, 200 mM NH4Cl, 150 mM TEOA, 10 mM trime-
Fig. 7 Effect of LED illumination and light intensity in photocatalysis thylpyruvate acid, 1 g Lꢂ1 resting cells, 5 mM photocatalyst, and a light
reaction. (A) Illumination condition affects photocatalysis; and (B) intensity of 2.29 mW cmꢂ2. Blank represents the reaction system
influence of illumination intensity on photocatalysis.
without photocatalyst.
37824 | RSC Adv., 2020, 10, 37820–37825
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
frequency (TOF) increased by 19-fold versus blank (Fig. 8). The
cofactor-recycling frequency (TOF) is dened as the total turn-
References
over number of NAD+ per hour (TTn per h), in which TTn ¼ [mol
1 F. Hollmann, I. W. C. E. Arends and D. Holtmann, Green
Chem., 2011, 13, 2285–2314.
(product or substrate)][mol NAD+]ꢂ1
.
However, the substrate
22
conversion rate of CdS QD photocatalytic reaction was 70% and
the TOF was increased by 14-fold. The photocatalytic efficiency
of CdS-NFs-A was not satisfactory with a substrate conversion of
only 55% and the TOF was increased by 11 times.
2 X. D. Wang and H. H. P. Yiu, ACS Catal., 2016, 6, 1880–1886.
3 X. D. Wang, T. Saba and H. H. P. Yiu, Chem, 2017, 2, 621–654.
4 S. S. Bhoware, K. Y. Kim, J. A. Kim, Q. Wu and J. L. Kim, J.
Phys. Chem. C, 2011, 115, 2553–2557.
CdS-NF nanomaterials formed a large network and porous
structure with CdS distributed on the network. Such a large
surface area and high affinity contribute to the binding of CdS
with cells, thereby improving the efficiency of electron transfer.
The diameter of the CdS QD nanoparticles was less than 5 nm,
but it tend to form large agglomerations that affected the effi-
ciency of the photocatalytic reaction. In addition, the CdS QDs
had weak affinity to cell membrane and just bound to the cell
with electrostatic interactions. The CdS-NFs-A formed shuttle-
shaped nanomaterials with a size over 500 nm, leading to
a recombination of electrons and holes, which reducing the
electron transfer efficiency.
5 O. Diwald, T. L. Thopson, T. Zubkov, G. Goralski,
W. S. D. Alck and J. T. Yates, J. Phys. Chem. B, 2004, 108,
6004–6008.
´
6 J. L. Guo, M. Suastegui, K. K. Sakimoto, V. M. Moody and
G. Xiao, Science, 2018, 362, 813–816.
7 H. L. Huang, Y. Jina, Z. G. Chaia and X. R. Gu, Appl. Catal., B,
2019, 257, 117869.
8 K. Kinastowska, J. Liu, J. M. Tobin and Y. Rakovich, Appl.
Catal., B, 2019, 243, 686–692.
9 K. A. Brown, D. F. Harris and M. B. Wilker, Science, 2016, 352,
448–450.
10 F. Chen, R. Zhou, L. Yang, M. Shi, G. Wu, M. Wang and
H. Chen, J. Phys. Chem. C, 2008, 112, 13457–13462.
11 Y. Huang, F. Sun, T. Wu, Q. Wu, Z. Huang, H. Su and
Z. Zhang, J. Solid State Chem., 2011, 184, 644.
12 M. R. Gholipour, C. T. Dinh, F. Beland and T. O. Do,
Nanoscale, 2015, 7, 8187–8203.
13 W. Wei, P. Q. Sun and Z. Li, Sci. Adv., 2018, 4, 9253–9260.
14 M. Mollavali, C. Falamaki and S. Rohani, Int. J. Hydrogen
Energy, 2018, 43(19), 9259–9278.
15 K. A. Brown, D. F. Harris and M. B. Wilker, Science, 2016, 352,
447–451.
16 W. W. Yu and X. G. Peng, Angew. Chem., Int. Ed., 2002, 41,
2368–2371.
17 D. H. Nam, S. H. Lee and C. B. Park, Small, 2010, 6, 922–926.
18 R. Zhai, Y. F. Yuan, F. L. Jiao, F. R. Hao and X. Fang, Anal.
Chim. Acta, 2017, 994, 19–28.
19 J. D. Cui, Y.-X. Feng, T. Lin, Z.-L. Tan, C. Zhong and S. Jia,
ACS Appl. Mater. Interfaces, 2017, 9, 10587–10594.
20 R. Banerjee, H. Furukawa, D. Britt, C. Knobler and
O. M. Yaghi, J. Am. Chem. Soc., 2009, 131, 3875–3877.
21 F. K. Shieh, S. C. Wang, S. Y. Leo and K. C.-W. Wu, Chem.–
Eur. J., 2013, 19, 11139–11142.
4. Conclusions
In this study, we developed a novel method to synthesize CdS-
NF nanoparticles in aqueous environment under ambient
conditions. The CdS-NFs have not only the excellent photo-
catalytic properties of CdS, but also the large surface area and
diverse porous structures of metal–organic framework.
Compared with conventional CdS synthesis methods, this
strategy is reliable, environmental friendly, and easier to
implement. The solubility of cadmium sulphide was greatly
improved in this reaction. In addition, CdS-NFs acted as the
photocatalyst and covalently bound with Escherichia coli to
catalyze the regeneration of cofactor, thereby promoting the
conversion of trimethylpyruvate acid to L-tert-leucine. Further-
more, CdS-NFs have a higher photocatalytic efficiency than CdS
QDs. In future, CdS-NFs will have broader prospects in the eld
of photocatalysis.
Conflicts of interest
The authors declare no conict of interest.
22 X. Gao, S. Yang, C. C. Zhao and Y. H. Ren, Angew. Chem., Int.
Ed., 2014, 53, 14027–14030.
ˇ
23 K. Matjaz, B. Irena and D. Anita, Ultrason. Sonochem., 2010,
Acknowledgements
17, 916–922.
This work was funded by the National Natural Science Foun-
dation of China (No. 21778018), Natural Science Foundation of
Shanghai (19ZR1412700), Fundamental Research Funds for the
Central Universities (No. 222201814035) and Research Program
of State Key Laboratory of Bioreactor Engineering.
24 D. L. Jin, G. H. Ye and J. W. Zheng, ACS Catal., 2017, 7, 5887–
5902.
25 G. H. Ye, Y. Y. Sun, Z. Y. Guo and K. K. Zhu, J. Catal., 2018,
360, 152–159.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37820–37825 | 37825