An Environment-Friendly Fertilizer Prepared by Layer-by-Layer Self

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An Environment-friendly Fertilizer Prepared by Layer-byLayer Self-assembly for pH-Responsive Nutrient Release Tao Li, Shaoyu Lü, Jia Yan, Xiao Bai, Chunmei Gao, and Mingzhu Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01425 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 26, 2019

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ACS Applied Materials & Interfaces

1

An Environment-friendly Fertilizer Prepared by Layer-by-Layer

2

Self-assembly for pH-Responsive Nutrient Release

3 4 5 6 7 8 9 10 11

Tao Lia, Shaoyu Lüa,*, Jia Yana, Xiao Baib, Chunmei Gaoa, Mingzhu Liua,* a

State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of China b Key Laboratory of Life-Organic Analysis of Shandong Province, School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273100, People’s Republic of China

12

ABSTRACT: Layer-by-layer (LBL) self-assembly based on natural polysaccharides is drawing

13

significant attention in various applications. However, its application in fertilizer is limited. In this

14

study,

15

environment-responsive release fertilizer with natural polyelectrolyte layers of chitosan and

16

lignosulfonate deposited on polydopamine-coated ammonium zinc phosphate. The morphology of

17

the fertilizer was evaluated by scanning electron microscopy, transmission electron microscopy

18

and atomic force microscopy. The composition and self-assembly process of the fertilizer were

19

characterized by elemental analysis, Fourier transform infrared (FTIR) spectroscopy, UV-vis

20

absorption spectroscopy, zeta potential analysis, thermal analysis, X-ray photoelectron

21

spectroscopy (XPS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES).

22

Excellent pH-responsive behavior was observed by the nutrients release results. In an alkaline

23

medium at room temperature, the nutrient release rate can be clearly accelerated compared with

24

acidic and neutral media. Moreover, pot experiments showed that fertilizer can effectively

25

promote plant growth. The pH-responsive environment-friendly fertilizer can control nutrient

26

release and avoid excessive release of nutrient, showing promising applications in modern green

27

and sustainable agriculture and horticulture.

28

KEYWORDS: LBL self-assembly, pH-Responsive release fertilizer, Natural polyelectrolytes,

29

Environment-friendly

LBL

electrostatic

self-assembly

technology

was

1

ACS Paragon Plus Environment

employed

to

prepare

an

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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INTRODUCTION

2

Fertilizer has played an important role in increasing crop yields, and will continue to be a

3

cornerstone of science-based agriculture to supply sufficient food for the expanding world

4

population. It is assumed that fertilizer will supply 70% of nutrients to satisfy the growth and

5

development of plants by 2020.1 However, traditional fertilizers tend to migrate into the

6

groundwater and atmosphere through volatilization, evaporation and leaching, due to their low

7

thermal stability and high solubility. These processes cause severe environmental problems

8

including eutrophication, acid rain and worsening global warming,2,

9

fertilizer utilization efficiency.4,

5

3

and also lead to low

To alleviate environmental problems and enhance fertilizer

10

utilization efficiency, slow/controlled-release fertilizers (S/CRFs) have been developed as an

11

effective strategy in modern agriculture.6

12

Compared to traditional fertilizer, S/CRFs have many advantages, such as high fertilizer

13

utilization efficiency, sustainable supply of nutrients, minimization of potential negative effects

14

and plant toxicity and low application costs.7-9 Encapsulated fertilizer granules coated with

15

natural10-12 or polymeric13-15 material is one common approach to prepare S/CRFs. However, the

16

technology of coating is limited by the complex production process.16 Therefore, it is necessary to

17

develop an ideal and effective coating approach.

18

In recent years, inspired by mussel chemistry, the multifunctional biopolymer polydopamine

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(Pdop), which has surface adherence, biocompatibility and low cytotoxicity17-19 and can be

20

prepared by self-polymerization on the surface of particles, has been applied in the field of

21

agriculture.20 Using the self-polymerization of dopamine hydrochloride, Jia et al.21 prepared a

22

Pdop-coated multielement compound fertilizer that exhibited excellent controlled-release behavior.

23

Although the release rate of nutrients can be controlled, nutrients content taken up by crops is

24

limited in different ambient conditions. To develop environmental-responsive fertilizers, the Jia

25

group reported polydopamine-graft-poly(acrylic acid)-coated CRFs with pH-responsiveness by

26

surface-initiated atom transfer radical polymerization (SI-ATRP).22 Our group prepared a fertilizer

27

with polymer brushes of poly(N,N-dimethylaminoethyl methacrylate) grafting from a

28

polydopamine-coated

29

controlled-release

fertilizer

behavior.23

core,

which

However,

exhibits

pH-

accompanying

2


and

temperature-responsive

environmental

pollution

and

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manufacturing costs emerge as a result of the non-degradability and non-renewability of synthetic

2

polymers, which have limited the widespread application of these environmental-responsive

3

fertilizers.

4

Recently, layer-by-layer (LBL) self-assembly technology has been shown to be generally

5

amenable to various materials including polymers,24, 25 biomaterials,26-28 inorganic substances29, 30

6

and supramolecular assemblies,31, 32 because of their simplicity and the chemical mildness of the

7

procedure.33 LBL self-assembly usually involves procedures where polysaccharides are

8

alternatively deposited on the surface of a solid substrate followed by rinsing with a solution to

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remove the physically adsorbed materials.34, 35 Chitosan is always selected as a positively charged

10

polysaccharide due to its good film-forming properties, biocompatibility, biodegradability,

11

polycationic nature and nontoxic properties.36-38 Sodium lignosulfonate, a byproduct from pulping

12

and papermaking waste liquid, can be used as a negatively charged polysaccharide for LBL

13

self-assembly due to its ionized functional groups.39,

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widely applied in various applications, such as films,41 chemical sensors,42 ultrathin membranes,43

15

optoelectronic devices,44 adsorbents45 and coatings,46 its application in fertilizer is limited,

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especially in environment-responsive fertilizer. Compared to reported methods (e.g., SI-ATRP)

17

for developing environment-responsive fertilizer, LBL self-assembly based on natural

18

polysaccharides has incomparable advantages, such as biodegradability, recyclability, low cost,

19

and renewability.

40

Although LBL self-assembly has been

20

Therefore, in this study, a simple and environment-friendly strategy using LBL self-assembly

21

was developed to prepare environment-responsive fertilizer for the first time. Dopamine adhered

22

to the surface of the nutrient substrate via self-polymerization. Subsequently, the natural

23

polysaccharides chitosan and sodium lignosulfonate were deposited on the Pdop layers by LBL

24

electrostatic self-assembly. The pH-responsive release behavior of nutrients was examined.

25 26

MATERIALS AND METHODS

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Material. Sodium lignosulfonate (SL) was purchased from Aladdin (Shanghai, China)

28

Biochemical Technology Co., Ltd. Chitosan (CS, deacetylation degree ≥90% and viscosity

29

50.0-800.0 MPa/s) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

30

Dopamine hydrochloride (Dop, 98%) was purchased from J&K Scientific Ltd. (Beijing, China). 3



1

All other reagents were analytical reagent grade and used directly. Water used throughout the

2

experiment was deionized.

3

Preparation of Ammonium Zinc Phosphate (ZnPN) and Polydopamine-coated ZnPN

4

(ZnPN@Pdop). First, ammonium zinc phosphate (ZnPN) was synthesized and then ZnPN@Pdop

5

was prepared by spontaneous oxidative polymerization of dopamine hydrochloride, according to

6

our previously reported procedure.23

7

Preparation of LBL Self-assembled Fertilizer. First, a chitosan solution and sodium

8

lignosulfonate solution (5 mg/mL each) were prepared. Chitosan (2.5 g) was dissolved in 500 mL

9

of a 1% (v/v) acetic acid solution. After the insoluble substance was removed by suction filtration,

10

NaCl was added to obtain a 0.5 M solution. Then, a 4 M NaOH solution was added to the mixture

11

to adjust the pH value to 5. For the sodium lignosulfonate solution, 2.5 g sodium lignosulfonate

12

was dissolved in 500 mL deionized water, and the insoluble material was removed by suction

13

filtration. Next, NaCl was added to obtain a 0.5 M solution. The pH value of the mixture was

14

adjusted to 5 by a 10% acetic acid solution.

15

ZnPN@Pdop (0.5 g) was dispersed in 40 mL deionized water, and then, 50 mL of the chitosan

16

solution (5 mg/mL) was added under mechanical stirring for 40 min. The unadsorbed chitosan was

17

removed by suction filtration and washing with deionized water. Then, the chitosan-coated

18

ZnPN@Pdop was dispersed in 40 mL deionized water. A 50 mL sodium lignosulfonate solution

19

(5 mg/mL) was added under mechanical stirring for 15 min. The unadsorbed sodium

20

lignosulfonate was removed by suction filtration and washing with deionized water. The

21

deposition procedure was repeated in the same way as mentioned above three times to obtain

22

self-assembled multielement compound fertilizer (SAMCF).

23

pH-Responsive Release Behavior of SAMCF. 0.3 g sample was dissolved in 5 mL aqueous

24

solutions and placed into a dialysis membrane bag (MWCO = 8000). Then, the dialysis bag was

25

immersed in 95 mL aqueous solutions with different pH values (4.0, 7.0, and 10.0), and incubated

26

at room temperature. At fixed time intervals, 5.0 mL of the release medium was collected and

27

replaced with fresh medium. After nitric acid-perchloric acid digestion, the released zinc (Zn) and

28

phosphorus (P) were determined using ICP-AES. In the meantime, the released ammonium (NH4+)

29

was evaluated by Nessler’s reagent colorimetric method.

30

Pot experiment. In brief, 0.2 g ZnPN, ZnPN@Pdop or SAMCF was spread on the surface of 150 4


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g dry soil in a pot. Afterwards, 10 g dry soil was added and two corn seeds were planted. Then, 30

2

g dry soil was put on the seeds. The pot was placed under 30-40% relative humidity condition at

3

room temperature. After germination, 15 mL water was sprayed every 2 days12. Values reported

4

were average of five experiments.

5 6

RESULTS AND DISCUSSION

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Preparation of SAMCF. Scheme 1 shows the LBL self-assembled process of SAMCF. First,

8

ZnPN@Pdop was prepared by adhesion and polymerization of dopamine hydrochloride on ZnPN.

9

Then, ZnPN@Pdop powder was soaked in a chitosan solution, rinsed with deionized water and

10

separated by centrifugation. Afterwards, the ZnPN@Pdop powder was soaked in sodium

11

lignosulfonate solution, rinsed with deionized water and separated by vacuum suction filtration.

12

The process was repeated until the desired layers were achieved. SAMCF was obtained after

13

drying at 50 °C overnight.

14 15

Scheme 1. Schematic of the LBL self-assembled process of SAMCF.

16 17

FTIR Analysis. The structures of sodium lignosulfonate, chitosan, ZnPN, ZnPN@Pdop and

18

SAMCF were confirmed by FTIR (NEXUS 670 FTIR Spectrometer, Nicolet USA), and the

19

results are exhibited in Figure 1A. In the spectrum of sodium lignosulfonate (Figure 1Aa), the

20

absorption peaks at 1598, 1515, 1118, 1039 and 916 cm−1 are attributed to the stretching

21

vibrations of aromatic rings, C-O-C of ether bonds, C-O of phenol and aliphatic hydroxyl groups

22

and the S=O bond, respectively. The absorption peaks of chitosan (Figure 1Ab) at 1655 and 1155

23

cm−1 are attributed to amide I and C-O-C bands of sugar rings, respectively. In the spectrum of

24

ZnPN (Figure 1Aa), the peak at approximately 3185 cm−1 is ascribed to the N-H stretching 5



1

vibration, whereas the absorption at approximately 1410 cm−1 is attributed to the N-H scissoring

2

vibration. In addition, absorption peaks at 1025 and 635 cm−1 are observed, which are attributed to

3

the P-O bond asymmetric stretching and the Zn-O bond vibration, respectively. For ZnPN@Pdop

4

(Figure 1Ab), peaks at 1605 and 1265 cm−1 are ascribed to the indole or indoline structures and

5

the C-N stretching vibration of the secondary amine in poly(dopamine), as reported previously.47

6

In the spectrum of SAMCF (Figure 1Ac), there is a broad and strong absorption peak at 3300 cm−1,

7

which is attributed to the -OH groups of chitosan and sodium lignosulfonate and the N-H

8

stretching vibration of ZnPN. The peaks at 1635, 1511, 1118, 1024, and 933cm−1 are attributed to

9

the stretching vibration of amide I, aromatic rings, C-O-C of chitosan skeleton,48 C-O of phenol

10

and aliphatic hydroxyl groups, and the stretching vibration of S=O,49 respectively. However,

11

compared with sodium lignosulfonate and chitosan, a redshift of the absorption peaks of SAMCF

12

was observed due to hydrogen bonding interactions among the polyelectrolytes and between the

13

polyelectrolyte and ZnPN@Pdop. The above results indicate that chitosan and sodium

14

lignosulfonate were deposited on the surface of ZnPN@Pdop without chemical reaction.

15 16 17 18 19

Figure 1. (A) FTIR spectra of sodium lignosulfonate (a), chitosan (b), ZnPN (c), ZnPN@Pdop (d), SAMCF (e); (B) UV-vis absorption spectra of SAMCF with different self-assembled layer: 6 layers (a), 4 layers (b) and 2 layers (c).

20

UV-vis Absorption Spectroscopy Analysis. The growth of the self-assembled layer on

21

ZnPN@Pdop was monitored by UV-vis absorption spectroscopy (Lambda35, Perkin-Elmer USA),

22

and the results are shown in Figure 1B. The characteristic wavelength of 280 nm is ascribing to

23

the absorption of the aromatic ring of sodium lignosulfonate. The peak amplitude at 280 nm

24

increases with the growth of the layer number. The results indicate that chitosan and sodium 6


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1

lignosulfonate have self-assembled alternately on ZnPN@Pdop substrate at each assembly cycle.

2

Similar results have been reported for other LBL self-assembled composites.50, 51

3 4

ζ-Potential Analysis. The LBL self-assembly process was characterized by Zetasizer (Nano ZS,

5

Malvern, Worcestershire, UK). The ζ-potential of ZnPN, ZnPN@Pdop, chitosan and sodium

6

lignosulfonate are shown in Figure 2A. The ζ-potential of ZnPN increases from -60.6 to -41.7 mV

7

after adhesion by Pdop. In addition, chitosan shows a positive potential (19.2 mV) and sodium

8

lignosulfonate shows a negative potential (-16 mV); thus, these polyelectrolytes can be used for

9

electrostatic self-assembly on the surface of ZnPN@Pdop. The ζ-potential of different layers of

10

self-assembled SAMCF is presented in Figure 2B. The ZnPN@Pdop substrate (approximately

11

-42.7 mV) was soaked continuously and alternately in chitosan and sodium lignosulfonate solution,

12

and the polyelectrolytes gradually deposit on the surface of the ZnPN@Pdop substrate. The

13

ζ-potential of the substrate changes, which is typical of LBL self-assembly of polyelectrolytes. It

14

is worth noting that the phenomenon of ζ-potential reversal did not appear as observed in regular

15

electrostatic LBL self-assembly.52 Similar behaviors were also reported previously,53,

16

indicated that the multilayer self-assembly could occur even without surface charge reversal, and

17

the most important parameter was a change in surface charge. It is assumed that the self-assembly

18

of polyelectrolytes depends on both electrostatic and hydrogen bonding interactions.55

54

which

19 20 21 22

Figure 2. (A) The ζ-potential of ZnPN (a), ZnPN@Pdop (b), chitosan (c) and sodium lignosulfonate (d); (B) The ζ-potential of the different layers of self-assembled SAMCF.

23

SEM and TEM Observation. The surface morphologies of ZnPN, ZnPN@Pdop and SAMCF

24

with 6 layers are presented in Figure 3. The SEM (ULTRA PLUS, Zeiss Company, Germany)

25

image of ZnPN in Figure 3a reveals a brick-shaped architecture, which is covered with a large 7



1

number of irregular cubic particles. At high magnification (Figure 3b), we can see that the

2

irregular cubic particles with a smooth surface aggregated and stacked together. After adhesion

3

and self-polymerization of dopamine hydrochloride on the surface of ZnPN, ZnPN@Pdop shows a

4

more regular brick-shaped structure and has a smooth surface with a small number of particles

5

(Figure 3c and 3d), which indicates the formation of a poly(dopamine) coating on the surface of

6

ZnPN. The SEM image of SAMCF with 6 layers (Figure 3e and 3f) presents a more smooth and

7

compact brick-shaped architecture compared with ZnPN@Pdop, which is due to the LBL

8

self-assembly of chitosan and sodium lignosulfonate on the surface of ZnPN@Pdop. The results

9

were further verified by the following TEM and AFM observations and elemental analysis and

10

TGA results.

11 12

Figure 3. SEM images of ZnPN (a and b), ZnPN@Pdop (c and d) and SAMCF (e and f).

13 14

To further confirm the successful LBL self-assembly, the samples were evaluated by TEM

15

(JEM-1200EX/S, Hitachi, Japan), and the results are shown in Figure 4. A thin polymer film of

16

approximately 30-70 nm thick is observed on ZnPN (Figure 4b and 4c), revealing the adhesion of

17

the Pdop layer. For SAMCF, there are two outer coating layers with thicknesses of approximately

18

50 nm and 70 nm from the LBL self-assembly of the chitosan and sodium lignosulfonate

19

polyelectrolytes on ZnPN@Pdop, as identified in Figure 4d and 4e. In addition, the atomic force 8


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1

microscopy (AFM, Asylum Research MFP-3D, USA) results shown in Figure S1 indicate

2

considerable alterations in the particle and surface height and roughness of the SAMCF after the

3

adhesion of Pdop and LBL self-assembly of polyelectrolytes,56 which is consistent with the TEM

4

results.

5

6 7 8 9

Figure 4. TEM images of ZnPN(a), ZnPN@Pdop (b and c), and SAMCF with 6 layers (d and e).

10

EDS and XPS Analysis. The distribution of elements in SAMCF was determined by scanning

11

electron microscope coupled with an energy dispersive X-ray spectroscopy analyzer (ULTRA

12

PLUS, Zeiss Company, Germany). The EDS spectra and EDS mapping of SAMCF with 6 layers

13

are presented in Figure 5. The EDS spectra of SAMCF demonstrate the presence of zinc,

14

phosphorus, nitrogen and, in particular, a large amount of carbon, which is consistent with the

15

elemental analysis of SAMCF. The EDS mapping indicates higher concentrations of zinc,

16

potassium, carbon and nitrogen. The existence of carbon atoms indicates that the natural

17

polyelectrolytes chitosan and sodium lignosulfonate have been deposited alternately on the surface

18

of ZnPN@Pdop via LBL electrostatic self-assembly.

19

The elemental composition and corresponding elemental valence states of SAMCF were

20

analyzed using XPS (Thermo Escalab 250Xi, USA), and the results are shown in Figure 6. O1s

21

(530.6 eV), N1s (397.8 eV) and C1s (282.2 eV) signals were observed. Moreover, typical Zn2p,

22

P2p and S2p peaks appear at approximately 1019.8 eV, 131.8 eV and 165.4 eV, respectively.

9



Page 10 of 32

1

These results are consistent with elemental analysis, as shown in Table 1. An increase in the

2

carbon content and decrease in the zinc, phosphorus and nitrogen content were observed, which is

3

because chitosan and sodium lignosulfonate as natural polymers, mainly contain the elements

4

carbon, hydrogen and oxygen. As the number of self-assembled layers increased, the amount of

5

polyelectrolytes deposited on ZnPN@Pdop gradually increased, and the content of carbon

6

increased. Meanwhile, the percentage of zinc, phosphorus and nitrogen was reduced

7

proportionally, which also indicated the formation of self-assembled polyelectrolyte layers.

8 9

Figure 5. EDS spectra and EDS mapping of the elements for SAMCF with 6 layers.

10

11 12 13 14 15

16 17

Figure 6. XPS spectra of SAMCF with 6 layers. Table 1. Characteristics of the samples. Samples

P (wt%)

Zn (wt %)

N (wt %)

C (wt %)

H (wt %)

ZnPN ZnPN@Pdop SAMCF-1a SAMCF-2b SAMCF-3c

23.54 20.13 16.17 13.25 11.59

34.23 29.43 26.31 24.88 20.19

7.40 7.17 2.34 1.65 1.15

0.00 2.34 3.78 6.05 11.28

2.04 2.24 1.98 2.05 2.24

a,b,c

SAMCF-1, -2 and -3 represent 2, 4 and 6 layers of the LBL self-assembly, respectively.

10


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Thermal Analysis. Figure 7A presents the experimental TGA (TG-STAPT1600, Linseis Inc.,

2

Germany) curves of all samples under a nitrogen atmosphere. According to Figure 7Aa, the

3

weight loss of ZnPN is 20.73%, which is attributed to the loss of adsorbed water at approximately

4

100 °C, a molecule of ammonia from 250 °C to 350 °C and crystal water from 350 °C to 450 °C.

5

After adhesion with Pdop (Figure 7Ab), 29.74% weight loss was observed. This result indicates

6

that Pdop has deposited on ZnPN, and its weight percentage is 9.01%. The TGA profile of

7

SAMCF has a higher weight loss up to 35.17%, which is because chitosan and sodium

8

lignosulfonate were alternately deposited on the surface of ZnPN@Pdop via LBL self-assembly.

9

In addition, SAMCF exhibits a three-stage weight loss process, as shown in Figure 7Ac. Absorbed

10

water and crystal water was lost until 140 °C and the deamination of ZnPN@Pdop occurred from

11

140°C to 300 °C. The decomposition of Pdop and the outer layer of self-assembled polyelectrolyte

12

account for the major weight loss from 300 to 800 °C. Comparing Figure 7Aa with Figure 7Ac,

13

the weight loss is approximately 14.29%, which is consistent with the carbon content of SAMCF

14

(11.28%) determined by elemental analysis, as shown in Table 1.

15

DSC (Sapphire DSC, Perkin-Elmer, USA) analysis results of ZnPN, ZnPN@Pdop and

16

SAMCF are shown in Figure 7B. For ZnPN (Figure 7Ba), there are two endothermic peaks at 125

17

°C and 192 °C, which are ascribed to the loss of free water and the deamination of ZnPN,

18

respectively. After coating with Pdop, there is a single, broad exothermic peak from 40-200 °C,

19

and the Tg is observed at 120 °C (Figure 7Bb). However, after LBL self-assembly (Figure 7Bc),

20

two endothermic peaks appear at 164 °C and 192 °C, which are attributed to the loss of water and,

21

the decomposition of polyelectrolytes and deamination of ZnPN@Pdop, respectively. The Tg of

22

SAMCF is observed at 145 °C. These results indicate that chitosan and sodium lignosulfonate

23

deposited on the surface of ZnPN@Pdop increased the Tg of SAMCF due to thermal

24

decomposition of the polyelectrolytes.

25

11



1 2 3 4

Figure 7. TGA curves (A) and DSC curves (B) of ZnPN (a), ZnPN@Pdop (b) and SAMCF (c) under nitrogen atmosphere.

5

pH-Responsive Release Behavior of SAMCF. The use of “smart” or “intelligent” fertilizer with

6

specific stimuli-responsive release behaviors has received increasing attention. In this study,

7

SAMCF was used as a fertilizer to evaluate its pH-responsive release properties over 28 days. The

8

results are shown in Figure 8.

9

Figure 8A presents the release behavior of phosphorus (PO43−) from SAMCF and

10

ZnPN@Pdop in solutions of pH 4.0, 7.0, and 10.0 at 25°C. Sustained and stimuli-responsive

11

release of phosphorus is observed. The phosphorus released from ZnPN@Pdop is 75.12%,

12

71.21%, and 83.15% at pH 4.0, 7.0, and 10.0, respectively. In acidic or basic conditions, the Pdop

13

layer is easily degraded and the multielement compound core is hydrolyzed, which accelerate the

14

effusion of the nutrients from the fertilizer core. For SAMCF, the release rate is slower than that

15

of ZnPN@Pdop under the corresponding pH conditions: the cumulative release rate of phosphorus

16

is 63.67%, 52.92%, and 77.65% at pH 4.0, 7.0, and 10.0, respectively. This result is due to the

17

additional diffusion barriers induced by the LBL self-assembly of polyelectrolytes on the surface

18

of the Pdop layer.

19

Similarly, the cumulative release rates of ammonium (NH4+) from ZnPN@Pdop are 81.20%,

20

73.21%, and 91.24% at pH 4.0, 7.0, and 10.0, respectively, as shown in Figure 8B. For SAMCF,

21

the corresponding cumulative release rates are 65.86%, 56.27%, and 74.95%, respectively.

22

The release rate depends on the character of Pdop and the disassembly of polyelectrolytes.

23

The Pdop layer, which has a great number of amino and phenolic groups, exhibits potentially

24

ampholytic or zwitterionic properties. The change in pH can affect both the amine and phenolic 12


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1

groups of the ion barrier formed with the Pdop layer. At low-pH conditions, the ion barrier allows

2

cations to pass but impedes anions, while it allows anions to pass and impedes cations at high-pH

3

conditions.57 The zwitterionicity of the Pdop layer can slightly accelerate the hydrolysis of the

4

ZnPN core. Moreover, the polyelectrolytes chitosan and lignosulfonate play significant roles in

5

pH-responsive controlled-release of SAMCF. In acidic and alkaline media, protonation and

6

deprotonation occur, respectively, causing the electrostatic repulsion of polyelectrolytes (Scheme

7

2). Therefore, the polyelectrolytes are more likely to disassemble and gradually migrate from the

8

surface of the fertilizer granule, which allows moisture to penetrate into the fertilizer core

9

facilitate nutrients dissolution.

10 11

Scheme 2. Scheme of pH-responsive behaviors of SAMCF in different solutions.

12 13

Figure 8C displays the pH-responsive behavior of zinc. The zinc released from ZnPN@Pdop

14

is 78.37%, 70.89%, and 83.26% at pH 4.0, 7.0, and 10.0, respectively. For SAMCF, the

15

cumulative release rates are 50.68%, 49.24%, and 47.39% in acidic, neutral, and alkaline solutions,

16

respectively. Note that the pH-responsive release behavior of zinc from SAMCF is different,

17

compared with the release behavior of ammonium and phosphorus. The cumulative release rate of

18

zinc from SAMCF is lower than that of phosphorus and ammonium, and the pH-responsive

19

release behavior is not significant. The reason for the differences is because of the adsorption and

20

chelation of zinc by lignosulfonate. There are sufficient functional groups on the backbone of

21

lignosulfonate, such as hydroxyl, aldehyde, keto, methoxyl, phenolic, and sulfonic groups. These

22

groups chelate with heavy metal ions, which affect their migration and transformation.58 In acidic

23

or alkaline solutions, the self-assembled layers on the surface of ZnPN@Pdop are more likely to

24

disassemble and slowly hydrolyze; the disassembled lignosulfonate facilitates the adsorption and

25

chelation of zinc ions that are released from the fertilizer cores, preventing some zinc ions from

26

diffusing completely and directly into the release medium. With sustained hydrolysis and

27

degradation of lignosulfonate, the adsorbed and chelated zinc ions are gradually released (Scheme 13



1

2). Therefore, the cumulative release rate of zinc from SAMCF is lower than that of ammonium

2

and phosphorus in the different pH solutions. Moreover, the cumulative release rate of zinc in

3

alkaline solution is slightly slower than in acidic solution, which is reversed in comparison to the

4

release behaviors of ammonium and phosphorus. This result may be due to the weak chelation of

5

the zinc ion by lignosulfonate in acidic conditions, limiting formation of the [Zn2+-lignosulfonate]

6

chelate compound.

7 8 9 10

Figure 8. pH-Responsive behavior of PO43− (A), NH4+ (B) and Zn2+ (C) from ZnPN@Pdop and SAMCF in solutions of pH 4.0, 7.0, and 10.0 at 25°C.

11

Effects of SAMCF on Seedling Growth. Seedling growth tests were carried out to determine the

12

applicability of SAMCF as a pH-responsive fertilizer. Loess soil (pH value is approximately 7.12)

13

from Lanzhou, Northwest China, was used for pot experiments. The corn seedlings were exposed

14

to bare soil, soil with ZnPN, soil with ZnPN@Pdop and soil with SAMCF, and their growth is

15

shown in Figure 9. It can be seen clearly that the corn seedlings involved with different treatments

16

show significantly different seeding lengths after the 20-day experiments. The corn seedlings

17

cultured with 0.3 g SAMCF exhibited an outstanding seedling length of 411 ± 3.21 mm compared

18

to those treated with ZnPN (344 ± 3.26 mm), ZnPN@Pdop (397 ± 2.15 mm) and bare soil (266 ±

14


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1

2.13 mm). Furthermore, the fresh and dry weight, stem height and root length of corn fertilized

2

with SAMCF are clearly improved (Table 2). These results show that SAMCF as a pH-responsive

3

fertilizer could effectively improve nutrients utilization efficiency, and significantly contribute to

4

crops growth. Note that the pH value of soil without any fertilizers increases from 7.12 to 7.65

5

after cultivation. This result occurs because irrigation water is weakly alkaline (pH=7.48).

6

Compared with the blank, the pH value of soil with SAMCF after cultivation is higher but similar

7

to that of ZnPN and ZnPN@Pdop. This is because that PO43− ions in fertilizer can also be

8

hydrolyzed slowly to generate more OH− in soil and increase the pH value of soil.

9 10

Figure 9. The images of corn after seeding for 20 days without treating (A) or treated with ZnPN

11

(B), ZnPN@Pdop (C) and SAMCF (D).

12 13 14

Table 2.The characterization of pot experiment.

Sample

15 16 17

Fresh weight

Dry weight

Seedling height

Stem height

Root length

(g)

(g)

(mm)

(mm)

(mm)

pH value of soil after cultivation*

Blank

0.85±0.14

0.11±0.02

266±25

105±11

55±5

7.65±0.04

ZnPN

1.09±0.21

0.16±0.03

344±18

114±8

60±9

7.72±0.04

ZnPN@Pdop

1.51±0.18

0.20±0.02

397±22

124±10

65±5

7.86±0.07

SAMCF

1.67±0.14

0.23±0.01

411±32

136±14

125±17

7.80±0.05

* The pH value of original soil solution is 7.12. Data are expressed as mean ± s.e. m. (cm) (n = 6). Means labeled in the same column are significantly different (p < 0.05)

18 19

Compared to conventional fertilizers and synthetic polymer-based stimuli-responsive 15



1

fertilizers, pH-responsive nutrients release behavior was achieved for the LBL self-assembly

2

fertilizer, and synthetic polymers are precluded, overcoming the shortcomings of harsh

3

preparation process and high manufacturing cost. Moreover, this environment-friendly fertilizer

4

based on natural polysaccharides exhibits biodegradability, recyclability, low cost, and

5

renewability, which are features that comply with modern green and sustainable agriculture and

6

meet the requirements of commercial production.59, 60

7 8

CONCLUSION

9

In this work, LBL electrostatic self-assembly technology was used to develop an

10

environment-friendly fertilizer with a Pdop inner layer and a pH-responsive polyelectrolyte outer

11

layer. The nutrient release results indicate that the fertilizer has different release profiles,

12

depending on the environmental pH, due to the disassembly and degradation of polyelectrolytes.

13

In addition, pot experiments indicate that the fertilizer can improve nutrients utilization efficiency

14

and significantly contribute to crop growth. Therefore, this inexpensive, readily available and

15

environment-friendly fertilizer may be widely used in modern green and sustainable agriculture

16

and horticulture. Furthermore, the LBL electrostatic self-assembly technology of natural

17

polyelectrolytes is promising in the development and application of novel smart fertilizers.

18 19

AUTHOR INFORMATION

20

Corresponding Authors

21

*S. Lü. Tel.: +86-931-8912387. Fax: +86-931-8912582. E-mail: [email protected].

22

*M. Liu. Tel.: +86-931-8912387. Fax: +86-931-8912582. E-mail: [email protected].

23

Notes

24

The authors declare no competing financial interest.

25 26

ACKNOWLEDGMENTS

27

The authors gratefully acknowledge the financial support of the National Natural Science

28

Foundation of China (grant no. 21875094, 51503091, 51273086, 51541304, 51603097), and the

29

Fundamental Research Funds for the Central Universities (grant no. lzujbky-2018-82).

30 31

REFERENCES 16


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An Environment-Friendly Fertilizer Prepared by Layer-by-Layer Self

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