Facile synthesis of yellow emissive carbon dots with high quantum yield and their application in construction of fluorescence-labeled shape memory nanocomposite

Journal Pre-proof Facile synthesis of yellow emissive carbon dots with high quantum yield and their application in construction of fluorescence-labeled shape memory nanocomposite Pei Yang, Ziqi Zhu, Xinghui Li, Tao Zhang, Wei Zhang, Minzhi Chen, Xiaoyan Zhou PII:

S0925-8388(20)30762-3

DOI:

https://doi.org/10.1016/j.jallcom.2020.154399

Reference:

JALCOM 154399

To appear in:

Journal of Alloys and Compounds

Received Date: 1 December 2019 Revised Date:

14 February 2020

Accepted Date: 16 February 2020

Please cite this article as: P. Yang, Z. Zhu, X. Li, T. Zhang, W. Zhang, M. Chen, X. Zhou, Facile synthesis of yellow emissive carbon dots with high quantum yield and their application in construction of fluorescence-labeled shape memory nanocomposite, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154399. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Facile synthesis of yellow emissive carbon dots with high quantum yield and their

2

application in construction of fluorescence-labeled shape memory nanocomposite

3

Pei Yanga,b, Ziqi Zhua,b, Xinghui Lia,b, Tao Zhanga,b, Wei Zhanga,b, Minzhi Chena,b*, Xiaoyan

4

Zhoua,b∗

5

a

6

210037, China

7

b

8

Abstract: Synthesizing carbon dots (CDs) with efficient long-wavelength emissions (i.e.,

9

yellow- to red light) generally suffer from sophisticated approaches, time-consuming process,

10

harsh conditions, and requirement of organic solvent; also, a further limitation of the resulting

11

CDs is relatively low quantum yield (QY) in aqueous solution. Herein, novel yellow emitting

12

CDs (Y-CDs) with a considerable QY of 62.8% were synthesized from a precursor

13

comprising resorcinol and o-phenylenediamine via a facile microwave method. To probe the

14

fluorescence mechanism, another typical resorcinol-derived CDs using ethylenediamine as

15

dopant were fabricated as well, showing strong green emission with an absolute QY of 60.6%.

16

Spectroscopic and structural characterizations indicated that the distinct redshift of green to

17

yellow emission depended on the dimension of conjugated sp2-domain and the content of

18

graphitic N heavily, while the excellent QY was highly related to the low proportion of

19

defective sp2 carbon cluster and high nitrogen content within CDs. Moreover, the Y-CDs were

20

confirmed to be capable of introducing additional crosslinking points in poly(vinyl alcohol) ∗

College of Materials Science and Engineering, Nanjing Forestry University, Nanjing

Fast-growing Tree & Agro-fibre Materials Engineering Center, Nanjing 210037, China

Corresponding author. E-mail Address: [email protected] (X.Y., Zhou); Tel: +86 025 8542 8506. ∗ Corresponding author. E-mail Address: [email protected] (M.Z., Chen); Tel: +86 025 8542 8040.

21

(PVA) polymer, which resulted in the Y-CDs-contained nanocomposite behaving superior and

22

tunable water-induced shape recovery performances. Importantly, since being labeled with

23

long-wavelength emission, the responsiveness of PVA/Y-CDs composite will contribute to its

24

versatile utilization in biology-relevant fields.

25

Keywords: carbon dots, yellow emission, fluorescence mechanism, nanocomposite, shape

26

recovery

27

1. INTRODUCTION

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Emerging from carbon materials, fluorescent carbon dots (CDs) have drawn heightened

29

attention in past decades because of their superior optical and chemical properties. Compared

30

with the conventional fluorescent materials, such as semiconductor quantum dots, organic

31

dyes and rare-earth phosphors, most of CDs reported so far mainly consisted of non-toxic C,

32

O and N elements, and are thus superior in the aspects of green synthesis, outstanding

33

photoluminescence (PL) performances, good water solubility, ease of functionalization, low

34

toxicity and excellent biocompatibility. Based on these intrinsic characteristics, CDs have

35

demonstrated a variety of potential applications, such as bioimaging [1,2], sensing [3-5],

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anti-counterfeit printing [6,7], photocatalysis [8-10], and optoelectronic devices [11,12].

37

Although a great number of synthesis strategies have been developed to fabricate CDs and

38

improve their PL quantum yield (QY), most of known CDs exhibit blue- or green-biased

39

emission behavior under excitation of ultraviolet (UV) light. Specifically, the PL spectra of

40

these CDs likewise follows similar trend in which the optimal emission peaks are located in

41

the blue and green light region, while further increasing excitation wavelength generally lead

42

to the attenuation of PL intensities. Such a limiting factor, which expressed as small

43

penetration depth, monotonous emissive light and inferior anti-interference property for most

44

currently available CDs, unquestionably restrict their wider applications, particularly in the

45

biology-related use, light-emitting devices, and information encryption. Therefore,

46

synthesizing CDs that emit luminescence with maxima in the long-wavelength region via a

47

facile approach, and achieving their high PL efficiency are still big challenges, because,

48

unlike to the semiconductor quantum dots, the tunable emission of CDs can be hardly

49

achieved by tailoring the particle size alone.

50

Attempts at achieving long-wavelength emissions of CDs have, to date, only been

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reported in few publications. In view of these researches, the multi-colored emissions in CDs

52

could

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concentration-dependent effect, and surface functionalization [13-17]. In addition, valuable

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utilization in bioimaging [18], fingerprint information recognition [19], promoting

55

photosynthesis [20], and constructing light emitting diodes [21,22] were also proposed on the

56

basis of these charming CDs. Furthermore, substantial efforts in these previous works were

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made to probe the underlying reasons for the long-wavelength emissions, and experimental

58

investigations have illustrated that the multiple PL properties strongly depend on the

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complicated chemical structures of CDs, involving conjugated sp2-domain, particle size,

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graphitization degree, heteroatom doping effect, surface-contained functional groups, etc.

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[1,11,18,21,23]. Nonetheless, these synthesized CDs with emission wavelength beyond the

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green light region generally suffer from several limitations: (1) the hydrothermal synthetic

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process is time- and energy-consuming; (2) the sophisticated separation or purification is

64

always necessary; (4) the tunable fluorescence is commonly realized via the solvothermal

be

realized

via

heteroatom

doping,

size

control,

solvatochromic

effect,

65

route or with the aid of dissolving CDs into organic solvent; (3) the QYs determined in water

66

is relatively low compared with the typical blue emissive CDs. Motivated by these issues,

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efforts should still be given to the significant work in this area for obtaining long-wavelength

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emissive CDs with high QYs, as well as exploring their novel application.

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As well known, shape memory polymers (SMPs) are able to experience large-scale

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shape changes with quick response upon exposing to a specific stimulus such as heat, light,

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solvents, electric, and magnetic fields. Structurally, stable cross-linked networks and

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switching segments are the two prerequisites for the construction of SMPs with shape

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recovery behavior [24]. Over the past few years, numerous SMP composites have been

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developed and exhibited their different stimuli-responsive performances, among these

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developed polymer-based SMPs, polyvinyl alcohol (PVA) as polymer matrix have caught

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increasing attention due to their larruping advantages, including nontoxic nature, easy

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processable, bio-compatibility and good mechanical properties. More importantly, PVA-based

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SMPs provide large possibilities to realize water-induced shape behavior, because water

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stimulus is much easier and milder for versatile utilizations. Combining the above merits, the

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function feature of PVA-based SMPs is more suitable to meet the requirement of many

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advanced applications, especially for biomedical engineering. In general, to improve the

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shape recovery properties of PVA-based SMPs, increasing researches have pointed out that

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the formation of extra hydrogen bonding interactions on polymer chains by immobilizing

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appropriate nanofillers in PVA matrix is an efficient way. In this regard, various fillers have

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been developed, such as polyaniline fibres [25], carboxymethyl cellulose sodium [26],

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aluminum oxide [27], and graphene oxide [28]. Commonly, for the PVA-based SMPs, the

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H-bond based crosslinking network induced by nanofillers is susceptive to water molecules,

88

and become weak in stiffness and mechanical stability with the increase of water content in

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SMP composite, which contribute more to the shape recovery of the SMPs. Such behavior

90

has been extensively investigated in previous reports, and the intrinsic reasons are highly

91

related to the plasticizing effect of water and the competitive hydrogen bonding interactions

92

within the crosslinking structure of SMPs [28,29]. However, it should be noted that the

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nonluminous characteristic of the water-responsive SMPs suffers difficulty in tracking their

94

recovery behaviors in dark environment, and therefore limiting their further development, in

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particular for their utilization in organism tissue. Benefiting from several nanometers in size,

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strong fluorescence, surface-contained multiple functional groups and excellent solvent

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dispensability, CDs are promising nanofillers that can be used to construct PVA-based SMPs

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featuring with stable fluorescence. Inspired by this charming function, a few studies have

99

reported on the potential benefits of water-induced shape memory of PVA/CDs composite

100

[29,30]. Despite these great efforts, the blue emission arising from these constructed

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PVA/CDs composites limit their proposed utilization in biomedicine field, because the

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biological matrix always behaves blue autofluorescence simultaneously once the

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CDs-contained SMPs was monitored using excitation light, such phenomenon is adverse to

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tracing the shape recovery when the constructed fluorescent SMPs are used in biological

105

system. Thus, it is of great and profound significant to construct water activated SMPs with

106

long-wavelength emission.

107

In this research, we synthesized yellow emitting CDs (Y-CDs) from a precursor

108

comprising resorcinol and o-phenylenediamine (OPD) via a facile microwave-assisted

109

hydrothermal method within just 10 min. The absolute PLQY of Y-CDs reached a level up to

110

as high as 62.76%, which would be comparable to most of blue- or green-emitting CDs, and

111

was much greater than the long-wavelength emissive CDs synthesized so far. Furthermore,

112

the PL emission mechanism for these resultant nanostructures was probed by comparatively

113

investigating the structure, morphology, and chemical composition. For expanding the

114

application of CDs, the synthesized Y-CDs were incorporated into PVA matrix to achieve

115

functionalized nanocomposite. Thanks to the formation of distinct H-bond based crosslinking

116

network induced by Y-CDs, the Y-CDs-contained PVA film demonstrated excellent

117

water-induced shape recovery behavior, accompanied with yellow fluorescence monitoring in

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dark environment. Additionally, this water-induced SMP constructed on the basis of Y-CDs

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and PVA showed tunable responsiveness by changing the environmental pH value. Such

120

inherent biological stimuli (water and pH), which together with the traced long-wavelength

121

fluorescence, will endow the PVA/Y-CDs SMP with diverse biomedical application potential,

122

such as smart biosensor, stimuli-responsive drug-release system and novel medical devices.

123

2. EXPERIMENTAL SECTION

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2.1 Materials

125

Resorcinol (≥99%), oPD (≥99%), and PVA (M.W.=145,000) were purchased from

126

Aladdin Co., Ltd. (Shanghai, China), HCl and NaOH were procured from Nanjing Chemical

127

Reagent Co., Ltd (Nanjing, China). Reagent grade of ethylenediamine (EDA) was obtained

128

from Shanghai Hansi Chemical Co., Ltd (Shanghai, China). All the reagents were used as

129

received without further purification. Ultrapure water was used throughout the whole

130

experiments.

131

2.2 Instruments and Measurements

132

The commercially available microwave instrument used in our experiment was produced

133

by Chemistry Electronic Microwave Company, (DISCOVER SP, USA). Transmission

134

electron microscopy (TEM) images was captured by a JEM-2100 UHR transmission electron

135

microscope (JEOL, Japan) with an accelerating voltage of 200 kV. Fourier transformed

136

infrared spectroscopy (FTIR) was investigated on a Vertex 80V FTIR spectrometer (Bruker,

137

Germany) with the scanning range from 4000 to 400 cm−1. The CDs solid samples were

138

measured by the KBr tablet method, while the PVA/CDs composites were tested using

139

attenuated total reflectance (ATR) mode. Surface charge measurement was performed on a

140

ZEN3600 Zeta Plus (Malvern, UK) instrument with a He-Ne laser source. X-ray

141

photoelectron spectroscopy (XPS) measurements were carried out on an AXIS Ultra DLD

142

electron spectrometer (Shimadzu, UK) with a monochromatic Al Kα X-ray source. UV-vis

143

absorption was performed using a Lambda 950 UV-Visible spectrophotometer (PerkinElmer,

144

USA). The PL performances were investigated by a F-7000 fluorescence spectrophotometer

145

manufactured by Hitachi, both the excitation and emission slits were 10 nm. Absolute PLQY

146

of CDs samples were measured by using an Edinburgh FLS1000 fluorescence spectrometer.

147

Time-resolved fluorescence decay curves were measured using a FLS1000 time-correlated

148

single-photon counting system (Edinburgh, UK). Wide-angle X-ray diffractograms (XRD)

149

analysis was realized by a Bruker D8 X-ray diffractometer adopting Cu radiation with Kα

150

wavelength of 0.1541 nm. Dynamic mechanical analysis (DMA) was performed on a DMA

151

242E (NETZSCH, Germany), the operating frequency was 1 Hz, and the test temperature

152

ranged from −40 to 120℃with a heating rate of 5°C min−1.

153

2.3 Microwave-assisted synthesis of CDs

154

In a typical procedure, 2 mmol resorcinol (220 mg), 2 mmol oPD (216 mg), and 200 µL

155

HCl were dissolved in 10 mL of ultrapure water for the synthesis of Y-CDs, the mixture was

156

then transferred into a 30 mL reaction tube, the glass tube containing starting solution was

157

heated to 180°C within tens of seconds and kept for an additional 10 min. After naturally

158

cooling down to room temperature, the reaction mixture was neutralized carefully by NaOH

159

solution. Subsequently, the resulting suspension was filtered by a PTFE syringe filter with

160

pore size of 0.22 µm to remove possible large particles, and then the filtered solution

161

subjected to dialysis (MWCO 800 Da) against ultrapure water for 48 h. This purified Y-CDs

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aqueous solution was collected for further use; also, the solid Y-CDs sample was obtained as

163

lightweight powder after freeze-drying. To probe the possible PL mechanism, 2 mmol EDA

164

(about 133 µL) was used to instead of oPD for preparing another typical nitrogen-doped CDs,

165

while the synthesis conditions were same as the preparation of Y-CDs. Because the green

166

fluorescence can be observed from these resultant CDs under a 365 nm UV lamp, this type of

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CDs synthesized using EDA as nitrogen dopant was labelled as G-CDs.

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2.4 Preparation of the PVA/Y-CDs composites

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In brief, 2 g of PVA powder was dissolved in 20 mL of ultrapure water, this mixture

170

suffered magnetic stirring at 90 ℃ for 2 h, then the obtained PVA solution was mixed with 25

171

mg of Y-CDs under continuous stirring for 1 h to ensure the thorough mixing of PVA and

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Y-CDs. Subsequently, the PVA/Y-CDs mixture was degassed using ultrasound for 30 min to

173

remove the air bubbles and then poured into a watch glass. Finally, the sample was dried at

174

60 ℃ in a vacuum oven until a constant weight was reached. Prior to use, the prepared

175

PVA/Y-CDs composite films were sealed in a desiccator to prevent from air moisture.

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2.5 Water-induced shape memory behaviors

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A bend-recovery test was performed to evaluate the shape recovery performance of

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PVA/Y-CDs composite in water. The composite film was cut into straight strip (40 mm length,

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5 mm width, and 0.2 mm thickness) and then bent to an angle (θ0) close to 180 at 80 ℃. After

180

that, the temporary angle (θi) generating from slight deformation was fixed by external force

181

during cooling. Finally, the V-shape specimen was immersed into water at room temperature

182

(25 ℃), and the change of bending angle (θf) with the prolongation of immersion time was

183

recorded. The shape recovery ratio (Rr) of the specimen was expressed as (θi − θf)/θi. For

184

monitoring in real-time, the recovery behavior of specimen was captured using a digital

185

camera.

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3. RESULTS AND DISCUSSION

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Benefiting from many advantages, such as environmentally friendly, simple in operation

188

and easy to implement, hydrothermal processing was widely used for the preparation of

189

nanomaterials over the past several decades and also deemed as an efficient way for the

190

sustainable development of CDs [31-34]. In addition, considering that microwave heating

191

does not only reduce the reaction time by several orders of magnitude, but also can suppress

192

side reactions, thus playing an important role in synthetic chemistry. Therefore, combing the

193

benefits of hydrothermal approach and microwave technology will provide a green and rapid

194

synthetic pathway for the production of CDs with long-wavelength emissions. As an optimal

195

example, Y-CDs were prepared facilely through a microwave-assisted hydrothermal method

196

within just 10 min, using the diluted HCl solution of resorcinol and oPD as starting materials.

197

It should be mentioned that the absence of HCl in the starting solution led to the aggregation

198

of resultant product which emitted weak fluorescence under UV lamp. Hence, the trace

199

amount of HCl can act as reaction promoter in solvent to facilitate the dehydration of the

200

precursors, and could be also conducive to the polymerization and carbonization of the

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molecule fragments, thus resulting in the formation of compact carbon cores with stable

202

luminescence centers. The possible promotion effect demonstrated above endowed the Y-CDs

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with a relatively bright fluorescence and high production yield. In the control experiment.

204

Note that the mole number of amino groups in EDA molecule was almost the same as that

205

contained within oPD, the added EDA, mixing with the HCl solution of resorcinol was

206

employed as the nitrogen precursor of G-CDs.

207 208

Figure1. Emission spectra of Y-CDs (a) and G-CDs (b) under excitation light of different

209

wavelengths, the insert photos are the corresponding CDs aqueous solution under UV light.

210

3D fluorescence plots of Y-CDs (c) and G-CDs (d).

211

Although an opportunity for doping same nitrogen content was provided, the resultant

212

Y-CDs and G-CDs demonstrated a distinct difference in emission behaviors. As shown in

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Figure 1, panel a illustrates that the Y-CDs were characterized by a typical

214

excitation-independent PL performance. Specifically, as the excitation wavelength increased

215

from 360 to 480 nm, the emission band peaked at 555 nm was insusceptible to this variation,

216

while the PL intensities increased firstly and decreased later. The excitation independency of

217

emission behavior could be mainly attributed to the homogeneous microstructure and surface

218

chemical states of the Y-CDs. Notably, despite pretty low PL intensity, CDs synthesized from

219

precursor comprising resorcinol and oPD without adding HCl showed a dominating emission

220

light at wavelength of about 555 nm (Figure S1), which was the same as the main emission

221

arising from Y-CDs. Remarkably, the excitation at shorter wavelength led to the emergence of

222

shoulder peak in blue-light region; also, this weak emission showed excitation-dependent

223

behavior which was basically similar to those widely observed in most of blue emissive CDs

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containing carbon topological defects [35]. Thus, it is reasonable to deduce that HCl in the

225

precursor of Y-CDs was beneficial to carbonizing intermediate products or structures.

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Consider this possible reason, we concluded that the incorporation of HCl was not the key

227

factor of resulting yellow fluorescence, but played a critical role in synthesizing CDs with

228

enhanced fluorescence and production yield. As for the G-CDs, the emission behavior was

229

examined at excitation wavelength ranging from 360 to 480 nm. Clearly, there were double

230

emission bands presented in the emission spectra obtained at shorter excitation wavelengths

231

(360–400 nm), taking the case of 360 excitation as an example, the emission peak of shoulder

232

band was located at 435 nm, while the main emission peak can be observed at 519 nm. With

233

the further increase of excitation wavelength, the shoulder peak disappeared gradually, and

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the main emission exhibited slight redshift in the wavelength range of 519 to 528 nm,

235

accompanied by the sharply increased PL intensity (Figure 1b). Moreover, both the Y-CDs

236

and G-CDs aqueous solution displayed a single narrow emission center in 3D fluorescence

237

plots. For the Y-CDs, the maximum emission center was located at 555 nm with an excitation

238

wavelength of 398 nm (Figure 1c), while the optimal excitation and emission of G-CDs were

239

observed at 485 nm and 529 nm, respectively (Figure 1d). Figure S2 of the supporting

240

information illustrates the pH-dependent PL performances of the two CDs samples, it is

241

apparent that both of the CDs samples demonstrated their weak fluorescence under acid

242

condition. For the low-dimensional materials like CDs, doping H+ was equivalent to injecting

243

holes into their structure, leading to change that the photogenerated electrons can be easily

244

captured by these extra holes [36,37]. As a consequence, the efficient radiative recombination

245

was weakened, which ultimately lowered PL intensities of the synthesized CDs. Furthermore,

246

measuring by an integrating sphere under 400 nm excitation, the Y-CDs exhibited the highest

247

QY of 62.8% among the reported CDs with long-wavelength emission in water (Figure S3).

248

In addition, the QY of 60.6% was also observed for G-CDs (Figure S4), which was also

249

superior over most of blue- or green-emitting CDs.

250 251

Figure 2. Absorption behavior (blue curve), fluorescence emission (red curve), and

252

fluorescence excitation (yellow curve) of Y-CDs (a) and G-CDs (b).

253

Figure 2 shows a comparison of absorption, PL excitation (PLE) spectra, and PL spectra

254

of the two types of CDs samples. The UV-vis absorption spectrum of Y-CDs exhibited an

255

obvious peak at about 276 nm, which is usually assigned to the π–π* transition occurred in

256

sp2 carbogenic domains [38,39]. Besides, in the lower energy region, a weak absorption band

257

at 400–500 nm was also distinct, which could be aroused by the defect states within CDs

258

structures or the large sized conjugated sp2-domain in the nitrogen-rich multilayer

259

graphene-like cores [19,21,38]. As seen in Figure 2a, it is clear that the PLE spectrum of

260

Y-CDs was partially overlapped with the absorption curve in the visible region, indicating

261

that the defect states transition at the core edge could also contribute to the yellow

262

fluorescence of Y-CDs. Significantly, the slight overlap between absorption and emission

263

spectra in the long-wavelength region indicated that Y-CDs can hardly absorb some part of

264

the PL emission generating from luminescent centers, and was thus bestowed with efficient

265

fluorescence emission [21]. Unlike the Y-CDs, the absorption behavior of G-CDs only

266

demonstrated π–π* transition with a characteristic peak at around 272 nm, and showed no

267

overlapped phenomena with emission and excitation spectrum (Figure 2b). The fluorescence

268

decay dynamics for Y-CDs and G-CDs were also measured to understand the electronic

269

transition process in molecular level. As shown in Figure S5, both the PL decay spectra of the

270

Y-CDs and G-CDs in aqueous solution can be well fitted with a mono-exponential function,

271

the lifetime component was 1.55 ns for Y-CDs, and 2.28 ns for G-CDs. Based on the

272

pioneers' research, the mono-exponential decay kinetics imply that the two typical CDs

273

underwent similar luminescence processes and there was only one dominating radiative

274

transition channel accounting for their PL emissions [40,41].

275

The morphological observation was performed firstly to understand the difference in PL

276

properties of Y-CDs and G-CDs, as shown in Figure 3. it can be clearly seen from the TEM

277

image that the Y-CDs were nearly spherical with excellent dispersibility (Figure 3a).

278

Furthermore, the high-resolution TEM (HRTEM) images displayed well-resolved lattice

279

fringes (Figure 3b), the determined interplanar spacing of 0.21 nm was close to the d-spacing

280

of the graphene (100) planes, suggesting the crystalline nature of Y-CDs. In addition, as

281

observed from TEM image of the G-CDs (Figure 3c), these nanostructures also had a uniform

282

dispersion but exhibited relatively small particle size compared with Y-CDs. Clearly,

283

attributed to the same carbon source and synthesis strategy, the resultant G-CDs was also

284

featured with same crystalline nature as that for Y-CDs (Figure 3d), thus demonstrating their

285

same multilayer graphene-like core structures. The particle size distribution of each CDs

286

sample was investigated as judged from 100 individual particles, the corresponding

287

histograms are displayed in Figure 4, panel a illustrates that the dimension of Y-CDs ranged

288

from 3 to 10 nm with an average diameter of 6.1±1.2 nm, In contrast, the G-CDs had a size

289

distribution in the region of 1.5–5 nm and average diameter of 3.1±0.7 nm (Figure 4b).

290

Although the particle size distribution of Y-CDs and G-CDs were both relatively wide, it is

291

note worth that most of Y-CDs had a uniform diameter in a small range of 5–7 nm, while the

292

dimension of large proportion of G-CDs was concentrated below 4 nm. The larger particle

293

sizes of Y-CDs could be resulted from the large sp2-domain that was formed due to the

294

intense reaction of resorcinol and oPD in HCl solution. Thus, it is reasonable to conclude that

295

the sp2-domain take the dominant place within the core structure and further help to tailor the

296

emission properties of CDs.

297 298

Figure 3. TEM image and HRTEM image of Y-CDs (a–b), and G-CDs (c–d).

299 300

Figure 4. Histograms of the particle size distribution of Y-CDs (a), and G-CDs (b).

301

The surface chemical species of the synthesized nanostructures were explored by FTIR

302

assignment. As illustrated in Figure S6, the similar absorption bands can be observed in the

303

FTIR spectra of Y-CDs and G-CDs, revealing little difference in their surface-contained

304

functional groups. The characteristic stretching vibration bands of O−H, N−H, and C−H were

305

observed at 3421, 3238, and 2896 cm−1, respectively. Note that the emergence of absorption

306

peak at 2595 cm−1 should be attributed to the −SH vibrations [42,43], the detected S element

307

in CDs could come from sulfur-containing impurities in industrial HCl reagent. Moreover, the

308

sharp absorption peaks at 1624 and 1506 cm−1 corresponded to the C=N and C=C stretching

309

vibrations [11,13,16], it is worthy of note that the peak intensity of C=C bond within Y-CDs

310

was much higher than that within G-CDs, indicating that the Y-CDs contained relatively large

311

sized conjugated sp2-domain in their nanostructures. Meanwhile, the bands at 1410 and 1284

312

cm−1 were the characteristics of C−N and aromatic C−NH stretching vibrations [44], while

313

the typical peak located at 1149 cm−1 was related to the C−O bond. In addition, the

314

absorption band at 750 cm−1 in the FTIR spectrum of Y-CDs was much more pronounced,

315

which could be correlate with the C−Cl groups [45], suggesting the covalent bonding of

316

chlorine atoms in the carbon skeleton of Y-CDs. As for the G-CDs, the characteristic

317

absorption peak of C−Cl bonds was obscure for the G-CDs because their content could be at

318

a low level. We speculated that HCl could react with oPD through electrophilic reaction

319

during hydrothermal process, which facilitates Cl doping within the structure of Y-CDs in

320

some extent. Measurement of Zeta potential show that the Y- and G-CDs had negative surface

321

potentials of −12.8 and −0.19 eV, respectively. the negatively charged properties of the two

322

typical CD particles and their significant difference could be dominated or affected by

323

Cl-containing groups and amino groups. In spite of the distinction in Cl-related chemical

324

moieties, the Y- and G-CDs synthesized from resorcinol coupling with oPD or EDA were

325

mainly composed of polyaromatic structures as well as abundant N-related groups.

326

XPS findings were used to further examine the chemical composition of CDs samples

327

for detailed comparisons, as shown in Figure 5; also, the relative content of each chemical

328

bond within the CDs’ structures was also extracted, as tabulated in Table S1. The full spectra

329

of Y- and G-CDs exhibited five distinct peaks: C1s (285 eV), N1s (400 eV), O1s (533 eV),

330

Cl2s (269 eV), and Cl2p (198 eV), indicating their same element composition. Additionally,

331

the chemical composition of the Y-CDs was 69.8% C, 15.8% N, 6.2% O, and 8.2% Cl, while

332

for the G-CDs, the atomic percentage was 60.7, 15.1, 11.9, and 12.3% for C, N, O and Cl,

333

respectively. Further, the O to C ratio for Y-CDs and G-CDs was determined to be 8.88% and

334

19.60%, respectively, suggesting a relatively high degree of graphitization in Y-CDs. By

335

contrast, there was no obvious change can be found for N/C ratio, although the N precursors

336

of Y-CDs and G-CDs were different. The above data indicates that the Y- and G-CDs

337

incorporated almost the same amount of nitrogen element within their structures. In the

338

high-resolution of C1s spectra, the characteristic peaks of C=C/C−C (284.5 eV), C−N (285.6

339

eV), and C−O (286.4 eV) can be found in both of the two CDs samples. Particularly, the

340

C=C/C−C moieties within Y-CDs reached up to 33.67%, which is relatively high compared

341

with 19.86% of G-CDs, thus indicating that the Y-CDs could contain much more aromatic

342

rings. For O1s, XPS spectra can be converted into C=O (531.8 eV), C−OH/C−O−C (532.7

343

eV), and C−O (533.5 eV) groups [20]. Although the N content in Y- and G-CDs were closely

344

similar, several distinctions were presented in the N-related chemical forms. In detail, The

345

N1s band of Y-CDs was deconvoluted into three distinct binding energy peaks at 399.4, 400.4

346

and 401.8 eV, which are assigned to the pyrrolic N, graphitic N, and amino N, respectively

347

[18,46]. The XPS N1s spectrum of G-CDs also indicated the presence of pyrrolic N and

348

amino N, but without diagnostic signal of graphitic N occurring from G-CDs, thus

349

confirming the shortage of this chemical species in G-CDs. Besides, it is worthy to note here

350

that there was relatively more pyrrolic N can be found in Y-CDs than in G-CDs, as illustrate

351

in Table S1. Since the pyrrolic N can contribute to the π-conjugated system with a pair of

352

π-electrons, the large amount of pyrrolic N within Y-CDs could help to form more efficient

353

conjugated aromatic π systems [1], which in turn reveals that the Y-CDs contained much

354

larger sp2-domain than G-CDs. In a nutshell, for the synthesized Y-CDs, the sp2 carbon

355

content was at a high level, and the graphitic N was also formed due likely to the

356

participation of oPD in the CDs synthesis reaction. From several previous investigations, the

357

extended efficient conjugation length which commonly expressed as increased particle size is

358

likely to narrow the band gap resulting from π electron delocalization within the sp2 domain

359

[1,21,47]. According to Holá and co-workers’ research, graphitic N within the carbon lattice

360

of CDs can donate excess electrons into the unoccupied π* orbitals of a conjugated system,

361

resulting in the visible absorption and red-shift of emission [48]. Inspired from these results,

362

it is reasonable to conclude that the distinct excitation-independent yellow emission of Y-CDs

363

was contributed from both the large conjugated sp2-domain and graphitic N within the

364

carbogenic cores. In the case of G-CDs, their excitonic absorption was located in only the UV

365

region, which could be closely correlated with the relatively small sized sp2-domain as well

366

as the absence of graphitic N; also, such behavior indicated that the luminescent centers of

367

G-CDs, where fluorescence was generated from the recombination of electrons and holes,

368

mainly associated with the π-conjugated system. Moreover, several defect states induced by

369

N-related functionalities or increased O-containing groups could also existed within the

370

electronic structure of G-CDs, but had weak capacity to generate efficient electron transitions

371

because the sp2-domain had dominating effect on the electronic properties. Thus, the trivial

372

defect states of GY-CDs gave rise to shoulder emission at blue light region, while the small

373

sized conjugated sp2-domain was more likely to be responsible for the green fluorescence.

374

Based on the above-mentioned analysis, the formation of Y-CDs with high QY was

375

tentatively proposed. In our case, with the assistance of microwave irradiation, the precursor

376

underwent polymerization initially, which resulted in the production of numerous

377

intermediate aromatic compounds including complete sp2-hybridized carbon and irregular

378

carbon frameworks with carbon vacancy. As the reaction proceeds, intermolecular

379

dehydration was happened under the harsh condition, and was also accelerated due to the

380

introduction HCl in starting solution, which further resulted in the nucleation of CDs,

381

accompanied with efficient carbonization. Since the similar molecular fragments took part in

382

the intense reactions, the resulting nuclei grow uniformly and isotropically toward the carbon

383

cores. As a result, most of vacancy in carbon frameworks was eliminated, and the large

384

sp2-domain took the dominant place within the core part of CDs. During the reaction, a great

385

of N atoms was doped within the sp2 clusters, and also connected on the edge of graphitic

386

layers simultaneously.

387 388

Figure 5. XPS full survey spectrum and high-resolution XPS of the C1s, O1s, and N1s

389

spectra of the Y-CDs (a), and G-CDs (b).

390

It has been known that introducing additional cross-linked points in PVA matrix could

391

improve the shape memory performances of PVA using water as stimulus. Inspired by the

392

easy formation of hydrogen bonding interactions with the PVA chains, the Y-CDs with

393

surface-contained multiple functional groups were employed as nanofillers to construct

394

PVA/Y-CDs nanocomposite with fluorescence-labelled shape recovery behavior. Prior to

395

evaluating water-induced shape memory property, the optical performances of the

396

PVA/Y-CDs composite was investigated as shown in Figure 6. The PVA/Y-CDs composite

397

film with thickness of 0.2 mm demonstrated relatively low transmittance compared with pure

398

PVA film (Figure 6a), which was attributed to the incorporation of brunet CD particles in

399

PVA matrix and the light absorption behavior of Y-CDs. As expected, because the single

400

fluorescence source was supplied by Y-CDs, the as-prepared Y-CDs/CDs composite film also

401

demonstrated excitation-independent PL behavior as similar to the pristine Y-CDs (Figure 6b).

402

Clearly, the emission peak of the nanocomposite was located at about 533 nm, showing

403

blue-shift phenomenon in comparison with Y-CDs aqueous solution, as seen in Figure 6c.

404

The underlying reason for the shift of emission peak could be related to the fact that the

405

formation of hydrogen bonds between Y-CDs and PVA matrix changed the electron transition

406

channels within Y-CDs. Despite showing blue-shifted phenomenon, the as-prepared

407

composite film with strong yellow-green fluorescence still has great advantage in biomedical

408

filed once being bestowed with water-responsive shape memory property.

409 410

Figure 6. (a) Transmittance of pure PVA film and PVA/Y-CDs composite film. (b)

411

Fluorescence emission spectra of PVA/Y-CDs composite excited from 360 to 480 nm. (c)

412

Normalized optimal emission spectrum of PVA/Y-CDs composite and Y-CDs aqueous

413

solution.

414

Further investigation on the shape recovery performance of PVA/Y-CDs composite was

415

performed, as shown in Figure 7a. A quatrefoil shaped sample was prepared and deformed

416

through folding. It is observed clearly that all petals of the closed flower shape were able to

417

bloom by the water-induced recovery process within a short duration of around 60 s. This

418

behavior demonstrated that the Y-CDs-contained PVA film exhibited outstanding

419

water-induced shape memory property. More importantly, benefiting from the bright

420

yellow-green fluorescence arising from Y-CDs, the whole recovery process can be traced

421

under a commercial UV lamp (365 nm excitation). To further confirm the contribution of the

422

Y-CDs toward shape recovery performance, the water-induced shape memory effect of

423

PVA/Y-CDs was also quantitatively assessed by a classical bending-recovery test under

424

different pH conditions. As presented in Figure 7b, all the three composite specimens

425

displayed superior water-induced shape memory properties with recovery ratio over 92%,

426

while the response rate was dependent on the water pH heavily. It can be clearly seen that the

427

original straight sample can be recovered within 60 s upon immersing in water with pH value

428

of 3, when the water pH changed to 7, the duration of complete recovery of PVA/Y-CDs

429

composite was 120 s, with further increasing pH value to 11, the deformed strip recovered to

430

its original shape slowly after been immersed in water at room temperature for about 180 s.

431

However, as we can see, the shape recovery of pure PVA film after deformation met

432

tremendous obstacle, the recovery ratio was only 29.3% even after being immersed in water

433

(pH=7) for 180 s (Figure 7c). The above-mentioned results clearly suggest that the

434

PVA/Y-CDs nanocomposite was advantageous over the pure PVA in terms of shape memory

435

effect. In particular, the visual images in Figure 7d and e also demonstrated that the recovery

436

process of PVA/Y-CDs composite strip can be monitored under UV light of 365 nm with

437

respect to the straight sample of pure PVA. Hence, the incorporation of Y-CDs in PVA matrix

438

not only contributed more to the shape memory property, but also offered excellent

439

performance in the visualization of shape recovery process when the SMPs was used in a

440

dark environment.

441 442

Figure 7. (a) water-induced shape memory behavior of the quatrefoil shaped sample under

443

365 nm UV light. (b) Shape recovery ratio of PVA/Y-CDs nanocomposite under different

444

water pH conditions. (c) Shape recovery ratio of pure PVA in water. (d) Water-induced shape

445

memory behavior of PVA/Y-CDs nanocomposite strip under a UV lamp in dark environment,

446

and (e) recovery process of deformed PVA strip in natural light.

447

For better understanding the mechanism of superior shape recovery in PVA/Y-CDs

448

SMPs actuated by water, a systematic investigation on the effect of Y-CDs toward PVA was

449

carried out. As shown in Figure 8a, an obvious increase of storage modulus obtained from

450

DMA was observed, from 4195 MPa for pure PVA to 5836 MPa for PVA/Y-CDs composite.

451

Besides, the glass transition temperature (Tg), which is defined as the peak position of

452

maximum tan δ, showed similar tendency, i.e., the Tg of PVA-based SMPs markedly

453

increased from 41.5 ℃ to 49.0 ℃ with the introduction of Y-CDs (Figure 8b). The above

454

experimental results are in good agreement with pioneer's investigation on the

455

thermomechanical properties of PVA/CDs composite [29], and also evidence that the Y-CDs

456

as functionalized fillers induced the formation of additional crosslinking points in PVA matrix

457

via interfacial interactions. The difference between pure PVA and PVA/Y-CDs composite in

458

crystallization was also examined by XRD analysis in Figure 8c. The 2θ position of the peak

459

in the X-ray scan for the nanocomposite was almost same as that of pure PVA, meaning that

460

the Y-CDs can only participated in the formation of physical crosslinking network. Note that

461

the prepared PVA/Y-CDs composite had a relatively low diffraction intensity compared with

462

pure PVA film, the decreased PVA crystallinity in the nanocomposite indicated that the chain

463

mobility of PVA was higher in the nanocomposite than in pure PVA film, such performance

464

could be highly related to the Y-CDs-induced extra crosslinking points in PVA polymer.

465

Taking into consideration that the nanosized Y-CDs possessed a great number of amino

466

groups as well as large surface area, it was easy to form hydrogen bonding interactions

467

between Y-CDs and PVA matrix, so that provided abundant extra crosslinking points in the

468

resultant nanocomposite. FTIR spectra were used to identify the hydrogen bonding

469

interactions, as displayed in Figure 8d. It is apparent that pure PVA showed typical stretching

470

vibration of O−H centering at 3287 cm−1; however, with the incorporation of Y-CDs, the

471

absorption peak of O−H stretching vibration shifted from 3287 to 3253 cm−1, the blueshift in

472

FTIR spectra fully revealed strong hydrogen bonding interaction between O−H in PVA and

473

N−H on Y-CDs surfaces. As another critical evidence, the PVA/Y-CDs composite possessed a

474

tensile strength of 57.4 MPa, which was 22.9% increase compared with the 46.7 MPa of the

475

pure PVA (Figure S7). The improved mechanical performance obtained in the CDs-contained

476

nanocomposite was more likely due to the intense hydrogen bonding interactions between

477

Y-CDs and PVA chains. According to the previous researches concerning water responsive

478

PVA-based SMPs [28], we proposed an underlying mechanism to clarify the improved shape

479

memory effect of PVA/Y-CDs composite. Since multiple hydrophilic groups in the

480

PVA/Y-CDs nanocomposite contributed to the affinity toward water molecules, the

481

competition of water-induced plasticity and Y-CDs-induced hydrogen bonding interactions

482

would occur within the structure of nanocomposite, such behavior weakened the H-bond

483

based physical crosslinking effect resulting from immobilized Y-CDs, and the plastication

484

from water penetration led to the active motion of soft segment and then the shape recovery

485

was happened. In addition, since the dissociation of hydrogen bonds which were formed

486

between amino groups and hydroxyl groups became easy as the water pH decreasing, the

487

shape recovery ratio was highly dependent on the acid base environment.

488 489

Figure 8. DMA of PVA/Y-CDs composite and pure PVA, (a) Storage modulus as a function of

490

temperature; (b) tangent delta as a function of temperature. (c) Comparison of XRD spectra

491

of PVA/Y-CDs composite and pure PVA. (d) FTIR spectra of PVA/Y-CDs composite and

492

pure PVA showing the changing of O−H stretching vibration.

493

4 CONCLUSIONS

494

In summary, with the assistance of microwave irradiation, yellow emissive Y-CDs with a

495

high quantum yield of 62.8% were facilely synthesized using resorcinol and oPD as starting

496

materials in HCl solution. To probe the underlying reason for the long-wavelength emission,

497

G-CDs with strong green fluorescence were also prepared following the same synthesis

498

strategy except using EDA as N-dopant to replace oPD. The systematic investigations on the

499

emission behavior, morphological structure, chemical composition indicated that the large

500

conjugated sp2-domain and graphitic N within the Y-CDs structure dominated their

501

long-wavelength emission. Furthermore, the considerable QY determined from the

502

synthesized CDs was mainly correlated with the complete carbon framework as well as

503

abundant N-containing groups within CDs, because the reduction of carbon vacancy in

504

carbogenic core, as well as the enriched electron density could promote more effective

505

radiative recombination from electron-hole pair. Since the Y-CDs can introduce the formation

506

H-bond based physical crosslinking networks within PVA matrix, the resultant PVA/Y-CDs

507

composite showed more excellent water-induced shape memory effect over pure PVA.

508

Furthermore, the shape recovery ratio of the nanocomposite showed increasing tendency as

509

the water pH increased, demonstrating that the tunable responsiveness of the

510

Y-CDs-contained nanocomposite can be achieved when using water as stimuli. More

511

importantly, labelling the PVA-based SMP with long-wavelength emission was beneficial to

512

monitoring shape recovery in dark environment, which will provide great possibilities for its

513

utilization in diverse biological fields, such as smart biosensor, stimuli-responsive

514

drug-release system and novel medical devices.

515

Acknowledgements

516

The authors are grateful to the National Natural Science Foundation of China (Grant No.

517

31870549), the Jiangsu Nature Science Foundation (BK20161524), the Program for 333

518

Talents Project in Jiangsu Province (Grant No. BRA2016381), the Postgraduate Research &

519

Practice Innovation Program of Jiangsu Province (KYCX17_839), the Advanced Analysis

520

and Testing Center of Nanjing Forestry University.

521

Conflict of Interest

522

Declarations of interest: none.

523

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Highlights • Yellow emissive carbon dots (CDs) with high quantum yield were facilely synthesized. • Emission behaviors of the CDs depended on carbogenic core and graphitic N content. • The CDs-containing nanocomposite showed fluorescence-labeled shape memory function.

Credit author statement Pei Yang: Conceptualization, Methodology, Formal analysis, Writing-Original Draft. Ziqi Zhu: Investigation. Xinghui Li: Investigation. Tao Zhang: Investigation. Wei Zhang: Investigation. Minzhi Chen: Conceptualization. Xiaoyan Zhou: Supervision, Writing-Review & Editing, Project administration.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: