Effect of Extraction Solvent on Removal of Excess Ligands in As-Synthesized Silver Colloidal Nanoparticles

Summary

Over 90% of chemical industries depend on catalysis. Nanocatalysis has never been more important than in today's society, responsible for optimizing waste water management, feedstock utilization, fuel cells, energy storage, and a plethora of other processes. Thus, a thorough analysis of these particles is both comercially and environmentally beneficial.

Ligands or capping agents are critical to the stabilization of colloidal nanoparticles. During synthesis, ligands are also often reducing agents and are usually added in excess so as to promote reaction kinetics. As a result, not all of them are adsorbed onto the particle interface; some remain free and un-adsorbed in the solution. These free residuals can have adverse effects on subsequent processes such as shell growth and ligand exchange. Thus, the removal of the free ligands is vital to manufacturing of nanoparticles. In this experiment, as-synthesized silver nanoparticles were purified by solvent extraction.

Different extracting solvents, including methanol, acetonitrile, dimethyl formaldehyde, and dimethyl sulfoxide were used. The ratio between the original solution and the extracting solvent was 1:1 (v/v). The samples before and after extraction were characterized with UV-Vis spectroscopy, transmission electron microscopy, and thermogravimetric analysis. UV-Vis confirmed that close to 100% of silver nanoparticles were recovered after extraction. The microscopy results confirmed that the shape and size of silver nanoparticles were maintained after the extraction. The thermogravimetric analysis results provided evidence that acetonitrile was the most effective solvent, yielding 62.9% extraction efficiency while the extraction with dimethyl formaldehyde and dimethyl sulfoxide provided only 25.3% and 22.0% in efficiency, respectively.

Question / Proposal

A better comprehension of nanocatalysts is necessary for academic, commercial, and environmental progress is our current society. They are responsible for waste water treatment, transportation fuel supply, vehicle emissions control, production and detection of low-toxic pesticide, and manufacturing of artificial fertilizers, pharmaceuticals, and polymers.

Nanocatalysts often see the use of capping agents, also known as ligands, in controlling particle growth and structure during synthesis. Many studies in the field have revealed both adverse and beneficial effects of these ligands regarding the catalytic performance of nanoparticles, but a conclusive quantitative analysis has yet to exist. Successful synthesis of nanoparticles usually require adding these ligands in excess amounts. Free ligands, however, can negatively impact subsequent steps such as shell growth and ligand exchange. The removal of both free and surface ligands is thus vital to achieving control over catalytic function of nanoparticles. Centrifugation, filtration, electrophoresis, extraction, and gel chromatography are several purification techniques, but each has their limitations. Liquid Liquid Extraction is a promising purification method for nanoparticles; it is a gentle process, and particles are unlikely to aggregate and coalesce irreversibly. A thorough analysis of the effect of each solvent on nanoparticle structure is extremely valuable to chemical industries, and this study aims to achieve provide that information.

Proposal:

This study aims to provide a thorough quantitative analysis of the effects of extraction solvent type on the removal of excess ligands in as-synthesized silver colloidal nanoparticles.

Hypothesis:

A thorough quantitative analysis can be provided given the silver nanoparticles undergo purification by liquid liquid extraction and are characterized by several analysis techniques including transmission electron microscopy, and thermogravimetric analysis.

Research

There are generally 3 methods by which nanoparticles are fabricated - through chemical, biological, or physical means. Chemical and biological synthesis of nanoparticles - the subject of this experiment - begins by reacting molecular precursors with reducing agents in the presence of capping agents, or ligands (which are often times the reducing agent themselves). Successful synthesis of nanoparticles usually require adding these ligands in excess amounts. As a result, some are adsorbed onto the precursors, forming nanoparticle complexes, while others remain free floating in the solution. Surface ligands can affect catalytic function in a variety of ways: fewer capping agents provides reagents greater access to the catalytic site, but can ultimately result in the aggregation of nanoparticles; excess capping agents, on the other hand, stabilizes the nanoparticles, but reduce catalytic activity. Free ligands, however, negatively impact subsequent steps such as shell growth and ligand exchange. The removal of both free and surface ligands is thus vital to achieving control over catalytic function of nanoparticles.

Different purification methods such as centrifugation, filtration, electrophoresis, extraction, and gel chromatography do exist. All of these processes, however, each has their own limitations. Centrifugation relies on different densities between the solvent and the nanoparticles and can be a potent separation method. However, it may cause aggregation, and fail to separate isolate components with similar densities. Electrophoresis is an effective way to extract ligands, but it can only undertake very small quantities at a time. Gel chromatography, like centrifugation relies on size properties, but it is labor intensive and time consuming at the same time. On the contrary, Liquid Liquid Extraction is a promising purification method for nanoparticle synthesis for both current and future researches. Unlike other extraction techniques, liquid liquid extraction is a gentle process; particles are thus unlikely to aggregate and coalescence irreversibly. During the extraction, 2 immiscible liquids form a separation layer. Under vigorous agitation, the components in one layer are driven into the other layer according to their polarity. Shaking and stirring are few of many ways to achieve this partition.

Although nanocatalysts are taking over the chemical industry, our understanding of the particles is still very obscure. There has been many attempts to understand the effect capping agents have on nanoparticles, but almost all have been qualitative. This study uses varying extraction solvents to achieve varying surfactant density after nanoparticle purification by liquid liquid extraction in order to provide a thorough quantitative analysis of the surface ligand density with the help of several analysis techniques (including UV-Visible spectroscopy, thermogravimetric analysis, and transmission electron microscopy), helping increase our ability to manipulate nanoparticle structure for industrial use. Processes such as fuel emissions control, wastewater management, and pesticide production can be lengthy and inefficient without catalytic aid. These processes involve distinct chemicals that respond differently to varying surface structures on nanocatalysts. Therefore, quantitatively characterizing the effects of extraction solvent type on as-synthesized particles can help chemical industries become more efficient, saving not only cost but also energy as well.

Method / Testing and Redesign

1. Synthesis

200 mL of hexane was stirred at 580 rpm and preheated until 50 ℃ in a round bottom flask. 0.2 g of silver acetate (precursor) and 10 mL of oleylamine (ligand and reducing agent) was then sonicated for 30 minutes and added to the flask. The temperature of the heating mantle was increased to 110℃, where the solution stabilized at 68 ℃ (boiling point of hexane). The resulting system was refluxed intermittently for a total of 36 hours, before retrievement and storage of its contents in a glass vial away from sunlight. The success of synthesis was confirmed by the mixture’s color transformation, which gradually turned from translucent to light yellow to dark brown, verifying the presence of silver nanoparticles.

2. Purification

Oleylamine was extracted from the silver nanoparticle solution (also termed "feed") by liquid - liquid extraction to obtain varying amounts of surfactant density. 4 extraction solvents were used in this study: methanol, acetonitrile, dimethyl formamide, and dimethyl sulfoxide. Extraction with acetonitrile, dimethyl formamide, and dimethyl sulfoxide was achieved by stirring 5 mL of solvent and 5 mL (1:1 ratio) of silver nanoparticle feed at 800 rpm for 6 hours. The top layer (termed “raffinate”) consisted mostly of hexane, with trace amounts of silver nanoparticles. The bottom layer (termed “extract) contained mostly the extraction solvent, with trace free floating ligands. The raffinate and extract were then separated into glass tubes for further analysis. The mixtures were allowed to settle into 2 layers for 30 minutes, before their volumes were measured. Purification with methanol, however, formed only one miscible phase when extracted in 1:1 ratio. Therefore, purification had to be accomplished with an extraction solvent to feed ratio of 2:1.

3. Sample Preparation and Analysis

0.1 mL of each raffinate was then extracted and diluted with hexane for UV-Vis analysis. The remaining raffinates were put into a rotary evaporator at 720 μatm in a 50℃ water bath for 30 minutes. The evaporated solutions were then lyophilized for thermogravimetric analysis. The samples before and after liquid liquid extraction were compared through UV-Vis analysis, thermogravimetric analysis, and transmission electron microscopy.  

                  

Rotary Evaporator

Safety Measures

  • Safety gloves were worn at all times while handling chemicals
  • All work was performed under the supervision of the laboratory professor

Results

The samples were analyzed with transmission electron microscopy to determine whether the extraction affected the morphology of the silver nanoparticle and prove that the purification technique was a gentle process. The microscopy images show that the silver nanoparticles before and after the extraction with DMF at 30 ºC were similar in shape, demonstrating that the extraction process did not interfere with nanoparticle structure. This notion was further proven when the size distribution of the particles after the extraction were plotted against that of the unpurified solution.

Additionally, each raffinate sample before and after purification was also analyzed under the UV-Visible spectrometer to ensure that no nanoparticles were leached into the extract. Percent recovery of nanoparticles were calculated according to the following formula:

(% Yield of Silver Nanopaticles) = (AFeedVFeedDFeed - AExtractVExtractDExtract) / (AFeedVFeedDFeed), where A is the absorbance obtained from UV-Vis Analysis at 436 nm, V is volume, and D is the dilution factor.

The amount of impurities, including surface and free ligands, remaining in the raffinate phase after purification was determined by thermogravimetric analysis. Weight loss occurred between 100 and 500 ºC for all 4 raffinates. This loss represents the thermal degradation of oleylamine, since silver nanoparticles are destabilized at a much higher temperature. The thermogravimetric curve reached a plateau at temperatures higher than 500 ºC, indicating that organic impurities were no longer present in the raffinate samples. ACN and Methanol purified raffinates experienced the least decrease in weight while DMF and DMSO showed the greatest increase. ACN was the best purifier, with a percent retention of 62.91%, while methanol, DMF, and DMSO, were able to extract 59.10%, 25.31%, and 21.99% of oleylamine respectively.

Furthermore, the differential thermogravimetric curve showed that the extraction solvent not only determined purification potency but also affected the properties of the solvent in which the silver nanoparticles were dispersed. The differential curve across all 4 graphs displays 2 peaks, from 100 to 250 ºC, and from 280 and 350 ºC. These temperatures conform to the degradation temperatures of free and surface ligands, with the former resembling free ligands and the latter representing surfactants. A comparison between the 4 curves shows that some free oleylamine became adsorbed onto the silver nanoparticle interface due to some change in properties of hexane. This is clearly visible in the DMF graph where the first peak is barely visible compared to the significantly large second peak as opposed to methanol whose two peaks are relatively equal in magnitude.

Conclusion

Over 90% of chemical industries rely on catalysis for production. As a result, nanocatalysts, with their unique nanoscale properties, are being given increasing interest in modern society and might eventually take over future production. These particles are key to a cost effective and energy efficient. Their applications are endless, whether it be for waste water treatment, transportation fuel supply, vehicle emissions control, production and detection of low-toxic pesticide, or manufacturing of artificial fertilizers, pharmaceuticals, and polymers. Many studies have tried to better understand nanocatalysts in order to utilize them for commercial use, focusing mainly on morphology and ligand density. However, most works have described only a qualitative aspect of the particles. This study aimed to provide a thorough quantitative description on the effects of solvent extraction of as-synthesized silver nanoparticles, and I believe that goal has been achieved.

Metal nanoparticles are often synthesized with excess ligands. These ligands can be adversarial and beneficial depending on the situation, but are considered impurities during manufacturing because they can affect post-synthesis steps such as ligand exchanges. Conventional methods such as centrifugation, filtration, and chromatography are lengthy and contain numerous limitations. As a result, we investigated an alternative - liquid liquid extraction - by extracting oleylamine from the silver nanoparticles solution dissolved in hexane. Oleylamine served as the reducing agent as well as capping agent of the reaction. Microscopy and UV-Vis spectrometry results ensured the reliability of the results. The extraction processes was considered to be mild because the silver nanoparticles were not damaged and maintained their original morphology after extraction, as shown by the transmission electron microscopy images of the samples from the extraction with dimethyl sulfoxide at 30 ºC. UV-Vis results confirmed that a negligible amount of silver nanoparticles were leached out during the extraction since the percent recovery of silver nanoparticles were close to 100%.

Thermogravimetric analysis showed that acetonitrile was the best purification solvent, with a percent oleylamine extraction of 62.91%, followed by methanol, dimethyl formaldehyde, and dimethyl sulfoxide, with  59.10%, 25.31%, and 21.99% of oleylamine respectively. Additionally, the thermogravimetric analysis shows us that the extraction solvent not only determined purification potency but also affected the properties of the hexane that dissolved the silver nanoparticles. Dimethyl sulfoxide and dimethyl formaldehyde had the greatest effect on the nanoparticle solvent, while acetonitrile and methanol did not show much deviation from the original feed curve.

This study establishes the extraction ability of different solvents for future references. Industries can make more informed choices when synthesizing nanocatalysts for optimizing reactions with a preferable ligand density. Moreover, this work emphasizes the effectiveness of solvent extraction as opposed to other conventional methods and lays a foundation for others trying to provide a quantitative characterization of other types of nanoparticles.

About me

My favorite subjects are chemistry, biology, and mathematics. Science and maths have always been my forte: I love performing labrartory experiments and challenging myself to difficult problems. I also absolutely adore classical music, having won several competitions both nationally and internationally over the past years. Coupling my fervor for science with my passion for music helpes me brainstorm innovations.

I want to major in chemical engineering at Stanford University, and help improve the standard of living in Thailand through the creation of non profit organizations that utilizes innovative technology. I am currently working on an NGO called eyesaver, which plans to distribute eye screening kits to elementary schools across Thailand with an instruction manual for teachers in order to prevent refractive errors and lazy eyes in children from worsening and becoming permanent. I already have the kit prototype done, and will plan to distribute 1000 kits in January 2019 in our first launch. (Eyesaver Website)

Winning this competition would mean the whole world to me. Not only will industrial processes take advantage of my research results to lower cost and improve the environment but my work will possibly also inspire others of my age to puruse science.

Health & Safety

Safety Guidlines

  1. Gloves must always be worn when handling chemicals
  2. Lab coats and safety gogggles mustbe worn at all times in labratory.
  3. All work must be done under the supervision of an adult.
  4. Do not smell punget chemicals.
  5. Dispose equipments and chemicals after usage into waste container

 

Supervisor / Lab Manager Contact Details:

Name: Nopphon Weeranoppanant Email: nopphon.we@eng.buu.ac.th  

 

Labratory Location

169 Long Had Bangsaen Road, Saen Suk, Chon Buri, Chon Buri 20131

 

Bibliography, references, and acknowledgements

Acknowledgements

I would like to take this opportunity to thank everyone who my experiment possible. First, my seniors at Burapha University, for always supporting me in in my journey; my adivsor, Dr. Jennings, for urging me to strive for the best things and for his invaluable advice; my professer, Nopphon Weeranoppanant, for providing supervision and knowledge, and for always being ready to help me out; and most importantly, my mom, for sacrificing so much for me, for taking 2 hour drives every week to send me off to the labratory, for always encouraging me to push forward, and for taking care of me every step of the way. This project would not have been possible without these people. They have my deepest regards.

Bibliography

[1] Y. Shen, M. Y. Gee, and A. B. Greytak “Purification technologies for colloidal nanocrystals,” Chemical Communications, vol. 53, no. 5, pp. 827-841, 2017.


[2] J. Hühn, C. Carrillo-Carrion, M. G. Soliman, , C. Pfeiffer, D. Valdeperez, A. Masood, I. Chakraborty, L. Zhu, M. Gallego, Z. Yue, M. Carril, N. Feliu, A. Escudero, A. M. Alkilany, B. Pelaz, P. D. Pino, and W. J. Parak “Selected standard protocols for the synthesis, phase transfer, and characterization of inorganic colloidal nanoparticles”, Chemistry of Materials, vol. 29, no. 1, pp. 399-461, 2016.


[3] W. J. Parak, T. Pellegrino, C. M. Micheel, D. Gerion, S. C. Williams, and A. P. Alivisatos, “Conformation of oligonucleotides attached to gold nanocrystals probed by gel electrophoresis”, Nano Letters, vol. 3, no. 1, pp. 33-36, 2003.


[4] W. W. Yu, and X. Peng “Formation of highquality CdS and other II–VI semiconductor nanocrystals in noncoordinating solvents: tunable reactivity of monomers”, Angewandte Chemie International Edition, vol. 41, no. 13, pp. 2368-2371, 2002.


[5] Y. Shen, N. Weeranoppanant, L. Xie, Y. Chen, M. R. Lusardi, J. Imbrogno, M. G. Bawendi and K.F. Jensen. “Multistage extraction platform for highly efficient and fully continuous purification of nanoparticles” Nanoscale, vol. 9, no. 23, pp. 7703-7707, 2017.


[6] Y. Shen, A. Roberge, R. Tan, M. Y. Gee, D. C. Gary, Y. Huang, D. A. Blom, B. C. Benicewicz, B. M. Cossairt, and A. B. Greytak “Gel permeation chromatography as a multifunctional processor for nanocrystal purification and on-column ligand exchange chemistry”, Chemical Science, vol. 7, no. 9, pp. 5671- 5679, 2016.


[7] W. Nan, Y. Niu, H. Qin, F. Cui, Y. Yang, R. Lai, W. Lin and X. Peng “Crystal structure control of zincblende CdSe/CdS core/shell nanocrystals: synthesis and structure-dependent optical properties” Journal of the American Chemical Society, vol. 134, no. 48, pp. 19685- 19693, 2012.


[8] Z. Li, Y. Ji, R. Xie, S. Y. Grisham, and X. Peng “Correlation of CdS nanocrystal formation with elemental sulfur activation and its implication in synthetic development” Journal of the American Chemical Society, vol. 133, no. 43, pp. 17248-17256, 2011.


[9] H. Hiramatsu and F. E. Osterloh “A simple large-scale synthesis of nearly monodisperse gold and silver nanoparticles with adjustable sizes and with exchangeable surfactants” Chemistry of Materials, vol. 16, no. 13, pp. 2509-2511, 2004.


[10] M. Seyhan, W. Kucharczyk, U. E. Yarar, K. Rickard, D. Rende, N. Baysal, S. Bucak, and R. Ozisik “Interfacial surfactant competition and its impact on poly (ethylene oxide)/Au and poly (ethylene oxide)/Ag nanocomposite properties” Nanotechnology, science and applications, vol. 10, pp. 69, 2017.