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Facile and Efficient Synthesis of Silver Nanoparticles based on Biore#nery Wood Lignin and its Application as the Optical Sensor Yuyuan Xue, Xueqing Qiu, Zewei Liu, and Yuan Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00578 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Sustainable Chemistry & Engineering

Facile and Efficient Synthesis of Silver Nanoparticles based on Biorefinery Wood Lignin and its Application as the Optical Sensor Yuyuan Xue,†, ‡ Xueqing Qiu†, ‡,* Zewei Liu† and Yuan Li†, ‡,* †

School of Chemistry and Chemical Engineering, South China University of

Technology, Guangzhou, China ‡

State Key Laboratory of Pulp and Paper Engineering, South China University of

Technology, Guangzhou, China * E-mail: [email protected] and [email protected]

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

Abstract: Fabricating silver nanoparticles (AgNPs) based on renewable energy sources is wildly exploited, owing to the sustainable synthetic strategy and versatile applications of AgNPs. Alkali lignin (AL), as the by-product from pulp mills, is a potential natural reducing agent. However, the synthetic methods of AL-based AgNPs (AL@Ag) still have drawbacks, such as unusual conditions, extra and high-cost purification processes. Here, a facile and efficient approach to synthesize and purify good-dispersing AL@Ag (17-27 nm) was presented, using Ag2O as the silver precursor and AL as both reducing agents and stabilizers in DMSO solvent. The maximum reduction capacity of AL to Ag+ was increased to 8 mM/g at room temperature, owing to the activation of both Ag2O and DMSO. Most conveniently, the product was effectively purified by easy centrifugation. The reducing mechanism and reaction behavior were also systematically studied. Meanwhile, AL@Ag maintained versatile applications of AgNPs and exhibited great potential as the colorimetric sensor and plasmonic resonance energy acceptor for Hg2+ and rhodamine B, respectively. Our work displayed a general and efficient method to prepare AL@Ag, which might provide a realizable perspective to the high-value utilization of lignin. Key words: Biomass; Ag nanoparticle; Ag2O; Lewis base; Organic-inorganic hybrid material; High-value utilization; Hg2+


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Introduction

Metal nanoparticles (MNPs), as silver and gold nanoparticles (AgNP/AuNP), have been attracting a continuous attention in the nanoscience and nanotechnology arena owning to their peculiar and exciting optical/electrical properties [1-12]. MNPs, especially for AgNP, possess localized surface plasmon resonance (LSPR) features with strong UV-vis absorption, resulting from the quantum confinement effects and interplay between surface and bulk effects. Versatile colorimetric have been extensively presented in previous works upon the property of LSPR as colorimetric probes [10-12]. Considering the potential versatility of AgNPs, kinds of synthetic methods of AgNPs were wildly studied [3, 8, 13]. Chemical reduction method was one of the most commonly used methods to synthesize uniform AgNPs, however, harsh reaction conditions and reducing agents (NaBH4, NaOH, thiols, etc.) added the cost of purification and raw material during the process of fabricating AgNPs [2,8,14-15].

Meanwhile,

non-environmental-friendly

stabilizing

agents,

like

cetyl-trimethylammonium bromide (CTAB) and other industrial surfactants, were wildly used [16]. Thus, intense researches have been directed toward the exploitation of green and efficient synthesis of AgNPs based on renewable energy sources [6-7, 12, 17-26]. Lignin is one of the most abundant components in plants. Annually, more than 70 million tons of alkali lignin (AL) is obtained from the pulp mill. Thus, more and more efforts are devoting to the high-value utilization of lignin [27-28]. Actually, the lignin-based noble metal nanoparticles have been extensively studied in previous years [29-32]. However, owning to the low reduction efficiency of lignin, many works only focused on its role as an abundant, inexpensive and nontoxic stabilizing material [29-32]. Thus, to enhance the reducing efficiency of lignin, the highly active silver precursor (Ag(NH3)2OH, instead of AgNO3, et al.) and drastic reaction conditions (NaOH activation, microwave radiation method and mechanochemical synthesis, et al) were used in recent works [17,21-26,33]. The corresponding synthetic methods,



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particle size, reducing capacity and purification process of AgNPs were shown in Table 1. Nevertheless, unusual conditions, complex and high-cost purification processes reduced the attraction of lignin-based AgNPs. The development of facile and efficient synthesis of lignin-based AgNPs was important.

Table 1 Synthetic methods, particle size, reducing capacity and purification process of AgNPs, using lignin as both reducing agent and stabilizer. Silver precursor

Reducing agent

Stabilizer

Ag(NH3)2OH

lignosulfonate

Ag(NH3)2OH

lignin

Condition

Lignin reducing capacity (mM/g)*

Size (nm)

Purification

Ref.

25 oC; 10 days

1.85

41

Not specified

17

6.00

~24

Dialysis

21

80 oC; NaOH; 2h

1.25

10-50

Not specified

23-24

Milling；95 h

1.84

14

Washing by water

22, 25

26

Microwave radiation; NaOH； 50-80 oC AgNO3

lignin mechanical

AgNO3

lignin decomposing

Ag(NH3)2NO3

wood

25 oC; 60 h

-

5-10

Washing by water

Ag2O

alkali lignin

25 oC; 12 h

8.00

~20

Centrifugation

This work

*The lignin reducing capacity was calculated from the optimum raw ratio in the literatures with the yield of 100 %.

Herein, a facile and efficient method to synthesize and purify AL-based AgNPs (AL@Ag) was demonstrated, using Ag2O as the silver precursor and AL as both reducing agents and stabilizers in dimethyl sulfoxide (DMSO) solvent. The maximum reduction capacity of AL to Ag+ was increased to 8 mM/g at room temperature, which was one of the highest reduction capacities, compared with other literatures [17,21-26,33]. In addition, the synthetic method in this work has advantages over the present methods. No harsh reducing agent or special conditions was used here and the good-dispersing AL@Ag was effectively purified by easy centrifugation. Moreover, the reducing mechanism and reaction behavior were systematically studied by in situ 1

H-NMR, fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD),


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X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), transmission electron microscope (TEM) and density functional theory (DFT) theoretical calculation. Meanwhile, the colorimetric sensor and fluorescence resonance energy transfer (FRET) system were designed based on the feature of surface plasmon resonance (SPR) and plasmon resonance energy transfer of Ag nano-platform, respectively.

Materials and methods Materials AL was supplied by Xiangjiang Papermaking Co. Ltd. (Yongzhou, China) and was used as received. The physicochemistry parameters of AL were shown in Table 2. The molecular weight, polydispersity (PDI), carboxyl and phenol hydroxyl contents of AL were 4300 Da, 2.76, 2.01 mmol/g and 1.69 mmol/g, respectively. Chloride salts of metal ion (Al3+, Cd2+, Na+, K+, Ca2+, Mg2+, Cr3+, Hg2+ and Co2+), sulfate salts of metal ion (Mn2+, Ni2+ and Zn2+), nitrate salts of metal ion (Li+, Mg2+, Fe2+, Cu2+, Ag+ and Pb2+), HAuCl4.3H2O, Ag2O, tris(hydroxymethyl)-aminomethane (Tris), sodium dihydrogen

phosphate,

sodium

hydrogen

phosphate,

DMSO,

N,N-Dimethylformamide (DMF), tetrahydrofuran (THF) and rhodamine B (RdB) were analytical grade. Deionized water (resistivity ≥ 18 MΩ/cm) was obtained from a water purification system and was used for preparation of samples and solutions as needed.

Table 2 Molecular weight, polydispersity (PDI), carboxyl and phenol hydroxyl contents of AL Sample AL

Mw/Da 4300

-COOH

Ph-OH.

(mmol/g)

(mmol/g)

2.01

1.69

PDI 2.76



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Figure 1 The schematic diagram for the synthetic method of AL@Ag. Synthesis and purification of AL@Ag A certain amount of Ag2O and 16 mg AL was added into 4 mL DMSO solution. After the mixture was stirred for 12 h at room temperature, the residual Ag2O was removed by centrifugation at a speed of 5000 rpm for 5 min and the purified AgNPs was obtained in the supernatant. The schematic diagram for the synthetic method of AL@Ag was shown in Figure 1. The mass ratios of AL/Ag2O were changed from 1:4 to 8:4, and the obtained products were numbered as 1#-5#, respectively, as shown in Table 3. Meanwhile, the powder of AL@Ag could be obtained by acidification at pH=3.

Table 3 Nanoparticle size, AgNP concentration and content of samples. AL

Ag2O

DMSO

AgNP Conc.

AgNP contenta

Size

(mg)

(mg)

(mL)

(mmol/L)

(wt %)

(nm)

1#

4

16

4

8

46.4

23.6

2#

8

16

4

16.4

47.0

20.0

3#

16

16

4

27.4

42.6

22.0

4#

24

16

4

31.6

36.3

21.9

5#

32

16

4

31.8

30.1

20.6

Sample

* a was calculated by Eq. (1) in the supporting information (SI).

Results and discussion Design and selection of Ag2O (silver precursor and catalyst) and DMSO (solvent and Lewis base) The facile and efficient method for the synthesis of AL@Ag was achieved from two parts: improving the reduction efficiency of lignin and simplifying the process of


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purification. It has been demonstrated that synthetic mechanism of lignin-based AgNPs is highly pH dependent that arise from the basic activation of phenol[23-24]. However, the needless ions of Na+ (from NaOH) and residual OH- would increase the complexity of product and the cost of purification. In order to avoid the introduction of inorganic ions, Lewis-base solvent of DMSO was chosen to replace the traditional NaOH. DMSO was a kind of important Lewis-base solvent, which played a good role of catalyst in the reaction activation of hydroxyl derivatives [35]. Then, DMSO was also a good solvent for alkali lignin (AL). Silver precursor was also an important aspect to be considered. Ag2O has been reported as an important catalyst in the reducing reaction of AgNPs [24, 35]. The by-product of Ag2O was water after the reduction. Meanwhile, Ag2O showed a very low solubility in DMSO, and the unreacted Ag2O could be facilely removed by centrifugation. Thus, Ag2O was chosen as the silver precursor in this work. To the best of our knowledge, this synthetic method of AL@Ag was first reported.

Figure 2 (A) The UV-vis spectra of AL@Ags (1-5#), after diluting 1000-fold with pure water. (B) The Zeta potential values of the AL@Ag containing solutions in different pH values. (C) Histograms of size distribution of samples (1-5#). Transmission electron microscopy (TEM) images of 1# (D), 3# (E) and 5# (F).

The Characterization of AL@Ag UV-vis spectrum was one of the most important techniques to investigate the



preparation of AL@Ag. As illustrated in Figure 2A, obvious SPR bands were centered at 416 nm, indicating the formation of AL@Ag. With the increase of mass ratio of AL/Ag2O from 1:4 (1#) to 8:4 (5#), the intensity of SPR bands of AL@Ag enhanced and the trend was in good agreement with the concentrations of Ag0, which were 8.0, 16.4, 27.4, 31.6 and 31.8 mmol/L, respectively. The AgNP concentration, content and size of samples were presented in Table 3. The mass content of Ag in the AL@Ag composite could reach up to 47 %. Meanwhile, AL showed a considerable reduction capacity to Ag2O, and the maximum reduction capacity of AL to Ag+ was 8 mM/g in AL+Ag2O+DMSO system at room temperature. The maximum reduction capacity of AL in this work was higher than those in the previous works and the corresponding data were listed in Table 1. The zeta potential was an important index to evaluate the stability of AL@Ag aqueous solution. The potential values of the AL@Ag aqueous solution were lower than -30 mV, when the pH value was higher than 4.0 (Figure 2B). AL@Ag could be stabilized by electrostatic repulsion with such low negative surface charges. The size and morphology of AL@Ag were determined by dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements (Figure 2C-2F). The average sizes of AL@Ag were found to be 23.6, 20.0, 22.0, 21.9 and 20.6 nm by DLS for the five samples from 1# to 5#, respectively (Figure 2C and Table 3). According to the literature, the size of AgNPs had great relationships with the molar ratio of Ag+ and reducing agents[1]. However, the sizes of AL@Ag particle nearly had little change, with the increase of mass ratio of AL/Ag2O. It was proposed that the reducing agent of AL was excessive during the reduction, comparing with the amount of Ag+ in DMSO solvent, because Ag2O showed a very low solubility in DMSO. Spherical/quasispherical shape of AL@Ag was observed by TEM with the particle size of 20 nm, which was in good agreement with DLS results (Figure 2D-2F).


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Figure 3 (A) XRD spectra of AL@Ags. (B) XPS spectra of AL and AL@Ag, illustration is the high-resolution XPS survey scan of Ag3d of AL@Ag.

The powder of AgNPs was further characterized using XRD and XPS measurements (Figure 3). Five peaks at 2θ= 38.25°, 44.42°, 64.53°, 77.59°, and 81.79° were observed in the XRD spectra, which were ascribed to the (111), (200), (220), (311), and (222) lattice planes of Ag, respectively (Figure 3A). The result was the same as JCPDS (No. 89-3722). Comparing with the XRD spectrum of Ag2O (Figure S1), there was no peak at 55°, indicating that no residual Ag2O existed in the composite. XPS was then used to further investigate the surface of AL@Ag. The main peaks at 572.7, 532.3, 374.3/368.3 and 284.6 eV were attributed to the Ag 3p, O 1s, Ag 3d and C 1s (Figure 3B). The high-resolution XPS spectrum of Ag 3d electron binding energy could only be divided into two peaks. The Ag 3d peak revealed the major component of Ag0 (368.3 eV for Ag03d5/2, 374.3 eV for Ag03d3/2) corresponding to the AgNPs. Moreover, this result also indicated that Ag+ did not exist in the composite and AL@Ag was effectively purified.



Figure 4 (A) In situ 1H-NMR spectra of AL with and without Ag2O powder in DMSO-d6. (B) FTIR spectra of AL and AL@Ag. High-resolution XPS survey scan of C1s of AL (C) and AL@Ag (D). (E) The suggested reaction equation between AL and Ag2O in DMSO solvent.

Structure Characterization of Lignin before and after Reaction As both the stabilizer and reducing agent, the structures of AL before and after redox reaction were further explored by 1H-NMR, FTIR and high-resolution XPS (Figure 4). In-situ 1H-NMR was used to study the structure of AL before and after redox reaction in N2 atmosphere to avoid absorbing water (Figure 4A and Figure S2). As shown in Figure S2, the signal at 3.3 ppm, which was ascribed to H2O, was obviously enhanced in the present of Ag2O. It revealed that H2O was expectedly generated in the redox reaction of Ag2O. Meanwhile, the previous research reported that the methoxyl group in AL structure would be removed in the redox reaction and the new generated aliphatic proton signals at 3.16 ppm, which was ascribed to the methyl of methanol, confirmed the same behavior in AL+Ag2O+DMSO system (Figure 4A) [21]. FTIR spectroscopy was further used to probe the chemical composition of the surface of AL@Ag (Figure 4B). The carbonyl (1645 cm-1) bands of AL@Ag was obviously stronger than that of the original AL. We proposed that the


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phenolic and alcoholic hydroxy in lignin were transformed to the quinone and carbonyl groups during the reaction, respectively. Quinone was the common oxidized product of phenolic derivative. Meanwhile, a new group, namely as carboxyl, was produced. The enhanced hydroxy (3450 cm-1) stretching band of AL@Ag was mainly from the carboxyl groups. The generated carboxyl was confirmed by the high-resolution XPS spectra of C1s, as shown in Figure 4C, 4D and Table S1. The C1s XPS spectrum of AL@Ag exhibited four peaks at 284.36, 285.45, 286.46 and 288.39 eV, which are attributed to C-C, C-OH, O-C-O/C=O and O-C=O groups, respectively. Comparing with the C1s XPS spectrum of AL, a new peak of O-C=O generated after the redox reaction. According to the results above, the suggested reaction equation in AL+Ag2O+DMSO system was depicted in Figure 4E.

Figure 5 The kinetics curve of AL+DMSO+Ag2O system with excess reducing agent of AL.

The study of reaction behavior The kinetics curve of AL+Ag2O+DMSO system was also studied with excess reducing agent of AL (Figure 5). The SPR bands of AL@Ag were monitored as a function of time. The first-order kinetics at the beginning (0-1.5 h) suggested a continuous and stable supply of Ag+ from Ag2O powder. With the consumption of Ag2O and AL, the reaction rate slowed down after 1.5 h and the second-order kinetics was observed (1.5-14 h). The reaction finished, when the catalyst of Ag2O was used up.



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Figure 6 (A) The absorption intensities at 416 nm of six compared samples (AL+DMSO+AgNO3,

DMSO+Ag2O,

AL+THF+Ag2O,

AAL+DMSO+Ag2O,

AL+DMSO+Ag2O, AL+DMF+Ag2O) after reaction for 4 hours (A0 is the corresponding initial absorption intensity at 416 nm), illustration was the corresponding UV-vis spectra from 250-700 nm. (B) Calculated electron cloud densities and redox potentials of coniferyl alcohol (the model molecule of AL) in THF and DMSO solvents, respectively. The schematic diagrams for the reaction mechanism of DMSO+Ag2O (C), AL+DMSO+AgNO3 (D), AL+THF+Ag2O (E), AL+DMSO+Ag2O (F), respectively.

Reaction Mechanism Analyses This synthetic method of AL@Ag was first reported. In order to understand the relationship between the roles of AL, DMSO and Ag2O, the compared experiments were designed. The intensity of SRP band and schematic diagram for the reaction mechanism were illustrated in Figure 6. Firstly, as shown in the illustration of Figure 6A, no SPR band was detected in DMSO+Ag2O system. It revealed that AgNPs could not be formed without the stabilizer and reducing agent of AL (Figure 6C). Meanwhile, the reducing capacity of acetylation of alkali lignin (AAL) significantly dropped off and the intensity of SPR band of AAL+DMSO+Ag2O system was only about one-third of that in AL+DMSO+Ag2O system (Figure 6A). The 1H-NMR and FTIR spectra of AAL were shown in Figure S3 and S4. The results indicated that a part of phenolic and alcoholic


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hydroxyl groups of AL were successfully blocked by acetylation. This phenomenon also indicated that hydroxyl group was the main reducing functional group of AL. Secondly, when the silver precursor changed to AgNO3, no AgNPs was formed and the reducing reaction did not occur (Figure 6A). It indicated that Ag2O not only acted as the role of silver precursor but also as the important catalyst for reduction process of Ag+ arising from metal-metal oxide interactions (Figure 6D) [24,35]. Finally, the Lewis base property of solvent was also indispensable. Comparing with DMSO, THF nearly has no property of Lewis base. When the solvent was changed to THF, no SPR band was detected in AL+Ag2O+THF system, and THF nearly showed little catalytic effect on the reducing agent of AL (Figure 6A). It indicated that Lewis-base solvent could enhance the reducing capacity of AL (Figure 6E and 6F). DMF has a stronger Lewis base property than that of DMSO. When the solvent was changed to DMF, a slight stronger SPR band was detected (Figure 6A). This phenomenon also confirmed the property of basic activation of Lewis-base solvents. To gain further understanding on the role of DMSO in the reducing system, density functional theory (DFT) calculation was used to study the change of electron cloud densities and redox potentials of the main reducing functional groups of lignin in DMSO and THF solvents, respectively. Coniferyl alcohol not only has the same reducing functional groups with AL but also is believed as a model molecule of lignin. Thus, coniferyl alcohol was used as the model molecule of lignin in the DFT calculation. As shown in Figure 6B, the electron cloud densities of alcoholic/phenolic hydroxyl groups of coniferyl alcohol in DMSO and THF solvents were increased from -0.6765/-0.7614 to -0.6947/-0.7782. The electron density of coniferyl alcohol was relatively localized in hydroxyl in DMSO solvent, comparing with that in THF solvent. Meanwhile, the redox potentials of coniferyl alcohol in DMSO solvent (-2.45 V) was obviously lower than that in THF (-2.08 V), which indicated that AL exhibited stronger reducing capacity in DMSO than that in THF. These results further indicated that DMSO could enhance the reducing capacity of AL. Thus, AL, Ag2O and DMSO together affected the reduction efficiency of AL@Ag and all of them were indispensable.



Figure 7 (A) UV-vis spectra of AL@Ag with different concentrations of Hg2+ (0-20 ppm), illustration was the corresponding images. (B) The linear relationship between the absorbance and Hg2+ concentration. (C) Histograms of Ix/I0 values of AL@Ag with different metal ions. (D)UV-vis spectra of AL@Ag solution (pH=7) with different metal ions (Ag+, Al3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+, 10-3 M). (E) Histograms of Ix/I0 values of AL@Ag mixed with Hg2+ and other different metal ions. (Ix is the absorption intensity at 400 nm after adding different metal ions.) (F) Schematic illustration of the detecting mechanism between AL@Ag and metal ions.

Colorimetric sensor Hg2+ is well known as one of the most toxic metals among various heavy metal ions, causing several developmental delays and health problems. Generally, in the presence of Hg2+, the AgNPs solution changes to colorless. Thus, AgNPs have attracted great attention as colorimetric probes for Hg2+. The sensitive detection of AL@Ag for Hg2+ was investigated in buffer solution (Tris-HCl, 0.05 M, pH=7). The color of AL@Ag solution changed from light-yellow to colorless immediately, with the addition of a known concentration of 10-4 M Hg2+. The image of AL@Ag solutions with different concentration of Hg2+ (0-200 ppm) was shown in Figure 7A. Hg2+ could be rapidly detected with AL@Ag by monitoring the


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color change with naked eye. The linear relationship between the maximum absorbance and Hg2+ concentration over the range of 0.2-20 ppm (Figure 7B) was observed, with the R2 of 0.9623 and 0.9916, respectively. Moreover, the SPR bands of AL@Ag were found to undergo a blue-shift, which indicated that the particle size of AL@Ag decreased with the addition of Hg2+ (Figure 7A). The phenomenon of decreasing particle sizes of AL@Ag was also observed by TEM. As shown in Figure S5, with the increase concentration of Hg2+, the size of AgNPs obviously decreased in the TEM images. Nearly no spherical AL@Ag was observed when the concentration of Hg2+ reached 10-3 M (Figure S5D). We supposed that a redox reaction between AL@Ag and Hg2+ was occurred, because Ag+/Ag (0.7996 V) showed a lower formal potential than that of Hg2+/Hg (0.851 V) (Figure 7F). In addition, Au3+ could also change the color of AL@Ag from light-yellow to colorless and the feature SPR band of AuNPs at 567 nm was detected in the mixture of AgNPs and Au3+ (Figure S6). This phenomenon further confirmed the reduction hypothesis above, owning to the low formal potential of Au3+/Au (0.93 V). Besides sensitivity, the selectivity of commonly metal ions (Ag+, Al3+, Ca2+, Cd2+, Co2+, Cr3+, Cu2+, Fe2+, Hg2+, K+, Li+, Mg2+, Mn2+, Na+, Ni2+, Pb2+ and Zn2+) was accordingly investigated. The UV-vis spectra were presented in Figure 7C and 7D. The yellow metal ions of Fe3+ and Cr6+ were not chosen in the selective experiment. As shown in Figure 7C, the absorbance at 400 nm with Hg2+ was much lower than other common metal ions and AL@Ag showed high selectivity toward Hg2+. Nevertheless, after the addition of Al3+, Co2+, Cr3+, Cu2+, Fe2+, Ni2+ and Pb2+ ions, the UV-vis absorption of AL@Ag was broadenedwith an obviously red-shift , owning to the aggregation of AgNPs (Figure 7D). The phenomena above would have little interference with the detection of Hg2+. Meanwhile, AL@Ag also showed very good antidisturbance in the presence of multifarious metal ions. The UV-vis spectra of AgNPs with the coexistence of Hg2+ and other common metal ions were shown in Figure 7E. The results found that other common metal ions did not interfere with Hg2+ detecting.



Figure 8 (A) Histograms of size distribution of AL@Ag in aqueous and PBS solutions. (B) Normalized UV-vis and fluorescent spectra of AL@Ag in aqueous/PBS solutions and RdB, respectively. (C) The adsorption of RdB on AL@Ag film. (D) Fluorescent spectra of AL@Ag-RdB composite in 0.1 M PBS buffers (0.2 mg/mL) after adding different volumes of AL@Ag PBS solution. Inset: Linear plot of I500/I576 to the volume of AL@Ag PBS solution (0-90 μL). λex = 400 nm. (E) The schematic diagram for the interparticle aggregation and role of energy acceptor of AL@Ag in salt solutions.

The FRET system of AgNPs-RdB Owing to the low extinction coefficient of spherical AgNPs at long wavelength (550-650 nm), the research of spherical AgNPs as energy acceptor for RdB was rarely reported. Nevertheless, the aggregation of AL@Ag with the red-shift SPR band was an operable method to solve this problem. The formation of AL@Ag-RdB composite was studied by DLS and QCM measurement. The DLS results of AL@Ag in aqueous and PBS solution were shown in Figure 8A. With the addition of PBS buffer solution, the aggregation of AL@Ag was obtained from electrostatic inhibition by salt. Comparing with the aqueous solution of AL@Ag, the particle size of aggregate AL@Ag increased from 20 nm to 1000 nm in the PBS buffer solution. Meanwhile, the UV-vis absorption at long wavelength was also


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obviously enhanced (Figure 8B). The increasing overlap (gray area) between the absorption spectrum of AL@Ag and the fluorescence spectrum of RdB was significant and beneficial for the energy transfer between AL@Ag and RdB. AL is a three-dimensional amorphous polymer consisting of many hydrophobic and hydrophilic groups, and could act as scaffolds for organic fluorophores. RdB belonged to cationic organic fluorescent molecule. The adsorption between RdB and aggregate AL@Ag would be realized via electrostatic as well as hydrophobic interactions. Quartz crystal monitor (QCM) measurement was a sensitive and effective sensor to explore the absorption behavior between AL@Ag and RdB (Figure 8C). 65.7 Hz of AL@Ag film was prepared on gold-coated substrate. After RdB aqueous solution was injected into the QCM-D flow module, the adsorption of RdB on the AL@Ag film was 4.4 Hz. The schematic diagram of AL@Ag-RdB system was shown in Figure 8E. Then, the fluorescent spectra of AgNPs-RdB composite was further explored (Figure 8D). With the addition of AgNPs PBS solution, the fluorescence of RdB at 576 nm was gradually quenched. It indicated that an effective energy transfer between RdB and aggregate AL@Ag occurred. Meanwhile, it was obviously that a new emission band at 500 nm gradually emerged with the addition of AL@Ag from 0-90 μL. This emission with broad bandwidth was from the surface of AL@Ag, namely as lignin, which has a broad fluorescence emission range and can be excited with both UV and visible light [36-38]. Unexpectedly, due to the inherent features of lignin, a ratiometric change of fluorescence was observed in the AL@Ag-RdB composite. The ratio of fluorescence intensities R (I500/I576) showed an observable enhancement (up to 49-fold) from 0.0167 to 0.814, with a good linear relationship. This result cannot be achieved based on the traditional capping agent, such as citrate, CTAB and cyclodextrin, because they are non-fluorescence.

Conclusions

In summary, a general and efficient method to synthesize and purify lignin-based AgNPs was presented and the maximum reduction capacity of AL to Ag+ was 8 mM/g



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at room temperature. The product was effectively purified by easy centrifugation, removing the residual Ag2O. It was also demonstrated that the Ag+-Ag2O interaction and the suitable Lewis base property of DMSO together opened the efficient synthetic method in this work and all of them are indispensable. Furthermore, the easy-fabricating AL@Ag maintained the high reactivity of AgNPs and could be employed as a sensitive and selective colorimetric sensor for Hg2+. Finally, the aggregation of AL@Ag, which was obtained by electrostatic inhibition, could act as an efficient plasmonic resonance energy acceptor for RdB and a ratiometric change of fluorescence was presented in the AgNPs-RdB composite system. Our work displayed a super-convenient approach for the synthesis of purified AL@Ag, which might provide a realizable perspective to the high-value utilization of lignin or other natural polyphenol compounds.

ASSOCIATED CONTENT Supporting Information The experiment methods of in situ 1H-NMR, DFT calculation, QCM-D measurement, reaction mechanism analyses, reaction kinetics measurement, procedures for colorimetric

sensing

and

AgNPs-RdB

composite,

measurements

of

AL

physicochemistry parameters and AL@Ag concentration; Instruments information of FTIR, TEM, XPS, XRD, DLS,

Zeta potential,

UV-vis

and

fluorescent

spectrophotometers; Synthesis of AAL; Figure S1-S6; Table S1.

Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (21436004), Science and Technology Program of Guangdong, China (2017B090903003, 201704030126), Natural Science Foundation of Guangdong (2017A030308012) and Pearl River S&T Nova Program of Guangzhou (201710010194).

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Table of Contents

Synopsis This super-convenient approach for the synthesis of purified AL@Ag provided a realizable perspective to the high-value utilization of lignin.


Page 24 of 25

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This super-convenient approach for the synthesis of purified AL@Ag provided a realizable perspective to the high-value utilization of lignin. 47x26mm (300 x 300 DPI)


Facile and Efficient Synthesis of Silver ... - ACS Publications

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