ZnS Quantum Dots (QDs) on the Root Epidermis of

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The retention of CdS/ZnS quantum dots (QDs) on the root epidermis of woody plant and its implications by benzo[a]pyrene: Evidences from the in situ synchronous nanosecond time-resolved fluorescence spectra method Ruilong Li, Haifeng Sun, Shaopeng Wang, Yinghui Wang, and Kefu Yu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b04258 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Journal of Agricultural and Food Chemistry

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The retention of CdS/ZnS quantum dots (QDs) on the root epidermis of woody plant

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and its implications by benzo[a]pyrene: Evidences from the in situ synchronous

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nanosecond time-resolved fluorescence spectra method

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Ruilong Li1,2,3, Haifeng Sun4, Shaopeng Wang1,2,3, Yinghui Wang1,2,3*, Kefu Yu1,2,3*

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1

School of Marine Sciences, Guangxi University, Nanning 530004, P.R. China

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2

Guangxi Laboratory on the Study of Coral Reefs in the South China Sea, Guangxi

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University, Nanning 530004, P.R. China

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3

11

China

12

4

13

China

Coral Reef Research Center of China, Guangxi University, Nanning 530004, P.R.

College of Environment and Resource, Shanxi University, Taiyuan 030006, P.R.

14 15 16 17 18 19

*

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University, Nanning 530004, P.R. China; Coral Reef Research Center of China,

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Guangxi University, Nanning 530004, P.R. China. E-mail: [email protected]

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(Yinghui Wang) and [email protected] (Kefu Yu)

Corresponding to: Yinghui Wang and Kefu Yu, School of Marine Sciences, Guangxi

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Table of Contents Graphic

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Abstract

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The retention of CdS/ZnS QDs on the epidermis has been confirmed to be one of

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core procedures during root uptake process. However, the retention mechanisms of

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QDs on the epidermis of woody plant were poorly understood for lacking of

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appropriate QDs quantitative method. In this study, a novel method for in situ

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determination of CdS/ZnS QDs retained on the root epidermis was established using

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synchronous nanosecond time-resolved fluorescence spectroscopy. No correlations

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between Kf values of Oleylamine-CdS/ZnS QDs retained on the epidermal tissues and

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the surface/bulk composition of mangrove root were observed (p> 0.05) due to the

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existence of endocytosis mechanisms during the QDs uptake processes. Moreover, the

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difference of the CdS/ZnS QDs in water and further translocated to xylem/phloem of

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root rather than the combination with cell wall/membranes were the predominant

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reason that caused the Kf values follow the sequence of PEG-COOH-CdS/ZnS QDs
A. corniculata

345

(0.41).

41

. These results

C-NMR data showed that the aliphatic carbon component accounted for 52.3 %,

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It can be clearly seen that there exists no strong correlation between Kf-oc values

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and the aromatic/aliphatic carbon of the bulk mangrove root with the coefficients (R2)

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of 0.64/0.03 (table 2). These divergent findings reveal that it is necessary to further

349

probe

the

relationships

of

root

surface

polarity

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and

the

retention

of

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Oleylamine-CdS/ZnS QDs. Fortunately, similar with the retention of PAHs

, the

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Oleylamine-CdS/ZnS QDs retained on the epidermal tissues of mangrove root

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ascends with the decreasing of the surface polarity ((N+O)/C) (table 2). However, no

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strong negatively correlations were found between the (N+O)/C values and Kf-oc

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values with coefficients (R2) of 0.22 (table 2). These two unexpected results indicated

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that the existence of other Oleylamine-CdS/ZnS QDs transport or binding with root

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epidermal tissues mechanisms made the compositions of mangrove root was not the

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sole determination factor of the retention of Oleylamine-CdS/ZnS QDs.

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As previously reported showed, no specific interactions including hydrogen

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bonds and electrostatic attraction were existed between Oleylamine-CdS/ZnS QDs

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and root cell/menbranes 43. Therefore, the existences of endocytosis, an invagination

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of the cell membrane for the uptake of extracellular materials (nm), was likely to be

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the reasons that the reduction of the correlations of Kf and chemical composition of

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root. To confirm our assumption, the effects of temperature on the retention of

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Oleylamine-CdS/ZnS QDs on the epidermal tissues were evaluated (figure S4).

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Results showed that the retention of Oleylamine-CdS/ZnS QDs at 298.15 K were

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markedly higher than 277.15 K (p< 0.05). Due to the passive apoplastic transport is a

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non-metabolic, non-energy consuming process, the retention of Oleylamine-CdS/ZnS

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QDs largely affected by temperature was an indicator that the transportation of this

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compound including biochemically processes, which, as Kettiger et al pointed out 44,

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was attributed to the apoplastic transport (refer the ‘‘Apoplastic versus symplastic

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transport’’ section) or endocytosis.

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Another evidence for the presences of endocytosis that affects the retention of

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QDs on the epidermal tissues of mangrove root come from the images of FLIM

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instrument,

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Oleylamine-CdS/ZnS QDs (green, 15-25 ns) and auto-fluorescence signal of root (red

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to yellow, < 10 ns) separately (Figure S5). In this figure, it can be clearly seen that

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although the dominant concentrations of Oleylamine-CdS/ZnS QDs located on the

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cell wall of epidermis, there still existed a small percentage of Oleylamine-CdS/ZnS

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QDs distributed on the intracellular of roots epidermis. As previously reports shown,

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the apoplastic transport (refer the ‘‘Apoplastic versus symplastic transport’’ section)

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or endocytosis were important pathways of neutrally QDs across the plant cell

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wall/membrane and thus the phenomenon described above reconfirmed our

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assumption

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Oleylamine-CdS/ZnS QDs retained on the mangrove root epidermal tissues were

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highly focused “stream” (aggregated) rather than patchily located in both longitudinal

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and transverse directions.

which

was

suitable

to

display

the

fluorescence

signal

of

41

. In addition, similar as parent/alkyl/heterocyclic PAHs, the

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3.3.2 The implications of charges of the QDs coating on its retained on the epidermal

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tissues of mangrove root

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Similar as Oleylamine-CdS/ZnS QDs, no positive/negative relationships were

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obtained

between

the

Kf-oc

values

of

PEG-COOH-CdS/ZnS

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PEG-NH2-CdS/ZnS QDs (Equilibrium time: 5 d) with different coating charges (The

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zeta potentials of -4.3 mV, -21.6 mV and 17.1 mV for Oleylamine-CdS/ZnS QDs,

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QDs

or

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PEG-COOH-CdS/ZnS QDs and PEG-NH2-CdS/ZnS QDs, respectively) and

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aromatic/aliphatic carbon of bulk root/surface polarity with the corresponding

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coefficients of 0.62/0.54/0.88 and 0.14/0.73/0.01 (table 2). More noteworthy was that

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the retention of ZnSe/ZnS and PEG-NH2-CdS/ZnS QDs on the epidermal tissues did

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not ascend with the decreasing of surface polarity of mangrove root. These results

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indicated that the surface coating charge of CdS/ZnS QDs, aside from the chemical

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composition of mangrove root, has potential ability to affect the retention of

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PEG-NH2-CdS/ZnS QDs on the epidermal tissues of mangrove root.

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As figure 3a showed, the Kf values of PEG- NH2-CdS/ZnS QDs on the epidermal

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tissues of K. obovata root were 7.4 (ng/spot)/(ng Cd /mg)n, which was a little higher

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than PEG-COOH-CdS/ZnS QDs (p> 0.05) but much lower than Oleylamine-CdS/ZnS

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QDs (p< 0.05). Similar results were obtained for A. marina and A. corniculata. Both

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of the cationic and anionic coating QDs easily dissolved in water and then majority of

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these kinds of QDs transport to the xylem/phloem tissues through root water and

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nutrients absorption pathways instead of retaining on the epidermal tissues 45, which

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leading to the Kf values of cationic and coating QDs much smaller than

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Oleylamine-CdS/ZnS QDs. Besides, the strong electrostatic attraction of positive

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charged QDs coating and the negatively charged root cell wall made the retention of

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PEG-NH2-CdS/ZnS QDs a little higher than PEG-COOH-CdS/ZnS QDs

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Overall, in this study, multi-factors, including the root composition, endocytosis

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and surface coating charges of CdS/ZnS QDs, were confirmed to be the factors that

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affect the retention of CdS/ZnS QDs on the epidermal tissues of mangrove root.

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3.4 The implications of B[a]P on the retention of CdS/ZnS QDs on the epidermal

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tissues of mangrove root

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The implications of PAHs on the retention of PEG-NH2-CdS/ZnS QDs on the

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epidermal tissues were displayed in figure 3b and 3c. The lower concentrations of

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B[a]P exposures (100 µg/L) inhibited the retention of PEG-NH2-CdS/ZnS QDs on the

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epidermal tissues of K. obovata root, with the Kf values decreased to 4.8

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(ng/spot)/(ng Cd /mg)n at 298.15 K. On the contrary, there existed almost no

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differences for the retained concentration of Oleylamine-CdS/ZnS QDs and

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PEG-COOH-CdS/ZnS QDs with and without the presence of B[a]P (p>0.05). Similar

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results were also obtained for A. marina and A. corniculata. When low concentrations

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of phenanthrene were presented, Yin et al confirmed that the depolarization of root

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membrane potential was obvious, and the magnitude of depolarization is in good

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accordance with the phenanthrene concentration uptake by root 24. Thus, the attractive

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force between the positively charged PEG-NH2-CdS/ZnS QDs located on the root

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surface and negatively charged root cell wall and membranes were weakened, which

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lowers the retention of PEG-NH2-CdS/ZnS QDs on the epidermal tissues of mangrove

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root. Meanwhile, the other physiological statuses of root did not affected by PAHs and

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the retained concentrations of Oleylamine-CdS/ZnS QDs and PEG-COOH-CdS/ZnS

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QDs kept constant.

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However, it should be noted that the Kf values of PEG-NH2-CdS/ZnS QDs retained

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on the epidermal tissues decreased to 3.1, 2.5 and 2.0 (ng/spot)/(ng Cd /mg)n for K.

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obovata, A. marina and A. corniculata, respectively, while the phenomenon was also

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an observer for Oleylamine-CdS/ZnS QDs and PEG-COOH-CdS/ZnS QDs at high

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B[a]P concentrations (500 µg/L, 298.15 K). Despite the depolarization of root

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membrane potential that inhibited the retention of PEG-NH2-CdS/ZnS QDs, as was

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reported by Dupuy et al

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endodermis of woody plant largely increased the possible adsorption sites for

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Oleylamine-CdS/ZnS QDs and PEG-COOH-CdS/ZnS QDs with the neutrally and

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negatively charge after 7 d exposure.

25

, the extensive deposition of suberin on exodermis and

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In this study, the S-NSFS method were established, which detection limits can

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reach as low as 1.2-2.0 ng/spot for the three kinds of CdS/ZnS QDs, providing a novel

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approach for sensitive and accurately in situ determination of trace concentration of

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the CdS/ZnS QDs retained on the epidermal tissues of root. More importantly, our

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studies firstly confirmed that the retention of CdS/ZnS QDs on the epidermal tissues

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was determined by multi-factors, which includes the composition of mangrove root,

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the endocytosis and the charge of the CdS/ZnS QDs coating, and further work showed

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that this fraction of CdS/ZnS QDs were largely affected by the PAHs in root. Overall,

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the findings of this work may be helpful in understanding the role of epidermal tissues

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in the uptake of CdS/ZnS QDs by woody plants.

456 457

Supporting Information

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The nanosecond time resolved fluorescence spectra of different kinds of QDs at

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optimal emission wavelength; The NSFS of the Oleylamine-CdS/ZnS QDs retained

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on the epidermal tissues of K. obovata root with the presence of B[a]P after filter out

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< 10 ns fluorescence signal (Figure S2); The rates of QDs retained on the epidermal

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tissues of mangrove root (Figure S3); The partition coefficients (Kf) of the QDs

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retained on the epidermal tissue of mangrove root at two different temperatures

464

(Figure S4); The FLIM images of Oleylamine-CdS/ZnS QDs retained on the

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epidermal tissue of mangrove root (Figure S5); Results of recovery experiment for the

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CdS/ZnS QDs retained on the epidermal tissues of mangrove root (Table S1);

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Integration results from

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Surface functionalities of the epidermal tissue of mangrove root acquired by XPS

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(Table S3)

13

C-NMR of different kinds of mangrove roots (Table S2);

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Acknowledgments

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The authors are grateful for financial support from the Natural Science

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Foundation of China (No. 91428203, No. 21507077, No. 41673105, No. 41273139)

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and the BaGui Scholars Program Foundation (2014).

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(a)

(b)

Fig. 1 The emission fluorescence spectra of 100 ng/spot Oleylamine-CdS/ZnS QDs on the epidermal tissues of mangrove root with (a) and without (b) filter the short lifetime auto-fluorescence signal (< 10 ns).

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5

1

Fig. 2. The S-NSFS of the Oleylamine-CdS/ZnS QDs retained on the epidermal tissues of K. obovata root with the presence of B[a]P after filters out the short-lived fluorescence signals (< 10 ns). Concentrations of Oleylamine-CdS/ZnS QDs adsorbed from 1-5 were 100, 200, 300, 400 and 550 ng/spot, respectively.

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(a)

(b)

(c)

Fig. 3. The partition coefficients (Kf) of the CdS/ZnS QDs retained on the epidermal tissue of mangrove root (a) without B[a]P and (b) 100 µg/L B[a]P (c) 500 µg/L B[a]P.

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Table 1. Analytical merits of the established method. Detection limita

Linear range Mangrove

QDs

Calibration curve

Correlation coefficient (ng/spot)

K. obovata

A. marina

A. corniculata

a

(ng/spot)

Oleylamine-CdS/ZnS

yb=9.2xc+127

15.5-2100

0.9826

1.5

PEG-COOH-CdS/ZnS

y=20.3x+101

21.0-3310

0.9711

1.2

PEG-NH2-CdS/ZnS

y=12.1x+203

34.5-1970

0.9909

2.0

Oleylamine-CdS/ZnS

y=14.7x+314

20.0-2700

0.9940

1.1

PEG-COOH-CdS/ZnS

y=22.6x+148

35.5-1700

0.9832

1.3

PEG-NH2-CdS/ZnS

y=15.5x+262

40.0-1260

0.9917

1.7

Oleylamine-CdS/ZnS

y=18.4x+186

15.5-4200

0.9944

1.5

PEG-COOH-CdS/ZnS

y=14.2x+210

28.5-1460

0.9809

1.8

PEG-NH2-CdS/ZnS

y=15.3x+226

36.0-2410

0.9922

1.6

detection limit of the method, which was calculated by 3SB/m, where ‘SB’ is the standard deviation of the blank, and ‘m’ is the slope of the calibration curve; b y

represents the S-NSFS intensity of CdS/ZnS QDs adsorbed onto the epidermal tissue of mangrove root. c x represents the concentrations of CdS/ZnS QDs adsorbed onto the epidermal tissue of mangrove root.

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Table 2 The correlation coefficients between the retained concentration of QDs and surface (O+N)/C or bulk compositions (aromatic carbon and aliphatic carbon) of mangrove root. QDs

Oleylamine-CdS/ZnS

PEG-COOH-CdS/ZnS

PEG-NH2-CdS/ZnS

a

Mangrove

Kf-oca

K. obovata

72.2

A. marina

74.5

A. corniculata

118.3

K. obovata

35.3

A. marina

24.1

A. corniculata

31.9

K. obovata

31.2

A. marina

23.9

A. corniculata

31.7

Fitting equation (S) b

R(S)2

Fitting equation (B) c

R(B)2

Fitting equation (A) d

R(A)2

ye=-0.0063xf+0.2859

0.22

y=-0.7117x+44.892

0.64

y=-0.0537x+55.844

0.03

y=-0.0023x+0.6803

0.62

y=-0.1451x+36.073

0.54

y=0.0712x+47.809

0.88

y=0.0075x+0.2659

0.14

y=-1.1383x+55.408

0.73

y=-0.0323x+55.013

0.01

Mean value of Kf-oc of six measurements, the units of Kf-oc was (ng/spot)/(ng Cd /mg)n; b Fitting equation (S) and R(S) represent the fitting equations and correlation

coefficient between Kf-oc value of CdS/ZnS QDs. and surface (O+N)/C, respectively; c Fitting equation (B) and R(B) represent the fitting equations and correlation coefficient between Kf-oc value of CdS/ZnS QDs. and aromatic carbon content of bulk root, respectively d Fitting equation (A) and R(A) represent the fitting equations

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and correlation coefficient between Kf-oc value of QDs. and aliphatic carbon content of bulk root, respectively; e y represents the value of surface polarity ((O+N)/C), bulk aromatic carbon or aliphatic carbon content of epidermal tissue of mangrove root; f x represents the Kf-oc value of QDs.

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