Exploring the Fundamentals of Microreactor ... - ACS Publications

Laboratory Experiment pubs.acs.org/jchemeduc

Exploring the Fundamentals of Microreactor Technology with Multidisciplinary Lab Experiments Combining the Synthesis and Characterization of Inorganic Nanoparticles Noémie Emmanuel,†,∥ Gauthier Emonds-Alt,†,‡,∥ Marjorie Lismont,*,§ Gauthier Eppe,*,‡ and Jean-Christophe M. Monbaliu*,† †

Center for Integrated Technology and Organic Synthesis, Department of Chemistry, ‡Laboratory of Inorganic Analytical Chemistry, Department of Chemistry, and §Biophotonics, Department of Physics, University of Liège, B-4000 Liège (Sart Tilman), Belgium S Supporting Information *

ABSTRACT: Multidisciplinary lab experiments combining microfluidics, nanoparticle synthesis, and characterization are presented. These experiments rely on the implementation of affordable yet efficient microfluidic setups based on perfluoroalkoxyalkane (PFA) capillary coils and standard HPLC connectors in upper undergraduate chemistry laboratories. Fundamental principles and concepts as well as practical tips for the rapid deployment of microfluidics are presented. Inline membrane separation, the segmented-flow regime, high-temperature experiments, and in-line analytical techniques are illustrated by the preparation of inorganic nanoparticles (silver, gold, and cadmium selenide or telluride) in microreactors. Besides microfluidics, analytical techniques for nanoparticle analysis are also illustrated. KEYWORDS: Upper-Division Undergraduate, Interdisciplinary/Multidisciplinary, Nanotechnology, Synthesis, Chemical Engineering, Analytical Chemistry, Aqueous Solution Chemistry, Microscale Lab, Laboratory Equipment/Apparatus

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dynamics,15 using examples ranging from purification techniques21 to analytical methods18−20 and preparative organic6−9,12,13 and inorganic chemistry.13,14 Capillaries made of perfluoroalkoxyalkane (PFA) are among the most popular solutions for the rapid and cost-efficient implementation of microfluidics in research laboratories.1,22 In contrast to PDMS and, to some extent, glass, PFA displays excellent resistance to harsh chemicals and mechanical stresses (up to 180 °C and 250 psi).1 PFA capillaries are also transparent, affordable, readily available with well-defined internal diameters ranging from 250 to 103 μm, and reusable. Complex microfluidic setups are easily constructed by combining PFA capillary coils and connectors at moderate costs and without complex micromachining procedures or equipment, therefore making microfluidic workshops accessible to groups of undergraduate students. Noble-metal nanoparticles (NPs) and semiconductor nanocrystals composed of group II to VI or III to V elements (a.k.a. quantum dots, QDs) have found numerous applications, and there is still an increasing research interest in developing new NPs and QDs or alternative routes to access high-quality NPs/ QDs.23−25 Just over the last two years, about three dozen

INTRODUCTION Microreactor technology is a blooming research area with a plethora of applications in chemistry, pharmaceutical sciences, and engineering1 as well as in physics2 and biology3 and as diagnosis tools for medical-oriented applications.4 Microreactor technology exploits the inherent properties of microstructured devices (a.k.a. microreactors) for processing materials. Fast transfer phenomena (mixing and heat exchange efficiency) and inherent safety are among the most prominent assets of microreactors.5 Typically, microreactors have internal diameters in the 102−103 μm range with internal volumes of up to 103 μL.5 They can be machined out of a variety of materials such as glass, metals, ceramics, or polymers, depending on mechanical, chemical, and biocompatibilities.5 Integrated systems combining pumps, microfluidic chips, and temperature and pressure controls are nowadays commercially available.5 Despite their practical aspects for laboratories and training,6,7 these integrated microfluidic devices are quite expensive, thus limiting their applications to small groups of students. Several articles have been published over the past decade in this Journal for introducing microreactor technology in early chemistry curricula using homemade microreactors constructed from glass, 8 − 1 2 poly(dimethylsilane) (PDMS),13−15 quartz,16 and paper.17−20 These articles illustrated some fundamental aspects of microfluidics, including the construction of microreactors7,9,10,13−15,17−20 and fluid © 2017 American Chemical Society and Division of Chemical Education, Inc.

Received: November 21, 2016 Revised: March 22, 2017 Published: April 7, 2017 775

DOI: 10.1021/acs.jchemed.6b00899 J. Chem. Educ. 2017, 94, 775−780

Journal of Chemical Education

Laboratory Experiment

articles dealing with NPs were published in this Journal.13,26,27 Despite this huge interest from the research and educational communities, only one example reported the microfluidic preparation of Au NPs in homemade PDMS chips.13 Herein we illustrate the implementation of PFA-capillarybased microfluidic assemblies for the preparation of NPs and QDs under rather unconventional conditions. Three workshops were successfully implemented with undergraduate students to illustrate some specific aspects of microfluidics and nanoparticle synthesis.28 Workshop #1 focuses on the preparation of Ag and Au NPs and illustrates the implementation of a segmented flow regime and in-line membrane separation. Workshop #2 deals with the preparation of cadmium selenide (CdSe) or cadmium telluride (CdTe) QDs at high temperature. Workshop #3 integrates an analytical application of Ag NPs for the detection of trace amounts of crystal violet (CV) in water by coupling microfluidics with surface enhanced Raman spectroscopy (SERS).

Gold Nanoparticles

The preparation of Au NPs from chloroauric acid (HAuCl4· 3H2O) and trisodium citrate (Na3C6H5O7·2H2O, TSC) was adapted from the procedure of Feng et al.13 (Figure 1).

Figure 1. Microfluidic setup for the preparation of Ag and Au NPs.

Heptane was used as an immiscible phase (Feed 1). Aqueous solutions of chloroauric acid (1 of 2 mM; Feed 2) and TSC (1 or 2 mM; Feed 3) were loaded in plastic syringes and injected at a flow rate of 25 μL min−1 each. The Au NP synthesis was carried out at different temperatures (e.g., 60 and 85 °C) under a pressure of 3 bar with different HAuCl4/TSC ratios (e.g., 1:1, 1:2, 2:1).

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EXPERIMENTAL OVERVIEW Each workshop is intended for a 4 h lab session, including the construction of the microfluidic devices, the experiments, and the characterization of the NPs (see the Supporting Information). The workshops can be implemented at the same time or independently. Before starting with experiments, students are required to get acquainted with the fundamentals of microfluidics, nanoparticle synthesis and characterization, and the construction of PFA assemblies.

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Cadmium Selenide and Cadmium Telluride Quantum Dots

CdSe and CdTe QDs were prepared according to two different methods (A and B) adapted from existing batch procedures (Figure 2).30,31 Method A used a high-boiling-point organic

EXPERIMENTAL DETAILS Figure 2. Microfluidic setup for the preparation of CdSe and CdTe QDs. Feed 3 is disconnected for method B.

Fabrication of Microfluidic Assemblies

The microfluidic setups were constructed with high-purity PFA coils (1/16″ o.d., 0.03″ i.d.) with internal volumes of 0.25, 0.5, 1.0, and 1.5 mL. The different coils can be rapidly exchanged to study the impact of the residence time without affecting the mixing efficiency. Standard HPLC-type ferrules, nuts, connectors, and T/Y-mixers were utilized. Liquid−liquid separation was performed with a hydrophobic membrane separator;1 alternatively, manual separation with a glass pipet could be utilized instead. Some experiments require a back-pressure regulator (BPR) to prevent the solvent from boiling. Microreactors were thermostated in a bath filled with a thermofluid. All reagents were injected using syringe pumps.

solvent (octadecene) as a growth solution. The Cd precursor solution was prepared by dissolving cadmium acetate dihydrate (Cd(CH3CO2)2·2H2O) and oleic acid in octadecene (Feed 1). The Se precursor solution was obtained by dissolving selenium in trioctylphosphine (Feed 2). Feed 3 consisted of pure octadecene. Feeds 1−3 were loaded in plastic syringes and injected at identical flow rates, which were adjusted to maintain a constant Cd/Se precursor ratio (1:7) and residence times ranging from 15 s to 7.5 min at 150 °C. Method B used water as the growth medium. The Cd precursor solution was prepared by dissolving Cd(CH3CO2)2·2H2O and 3-mercaptopropionic acid in degassed D.I. water, and the pH of the resulting cloudy solution was then adjusted to 9.3 (NaOH, 1 M) (Feed 1). Feed 2 was prepared by adding NaBH4 to a suspension of Se in degassed D.I. water. Feeds 1 and 2 were loaded in plastic syringes, and their flow rates were adjusted to maintain a constant Cd/Se precursor ratio (1:1) and residence times ranging from 15 s to 9 min at 100 °C. For instance, Feed 1 was delivered at 88 μL min−1 and Feed 2 at 22 μL min−1 for a residence time of 9 min (1 mL internal volume PFA coil). CdTe QDs could be prepared according to a similar procedure at higher temperature (150 °C) and pressure (4 bar) with longer residence times (30 min) using an aqueous Te precursor solution.

Silver Nanoparticles

The preparation of Ag NPs from silver nitrate (AgNO3) and sodium borohydride (NaBH4) was adapted from the procedure of Gavriilidis and co-workers.29 Heptane was used as an immiscible carrier (Feed 1). Solutions of AgNO3 (1 or 2 mM; Feed 2) and NaBH4 (2 mM; Feed 3) were prepared in deionized (D.I.) water, loaded in plastic syringes, and injected at a flow rate of 50 μL min−1 each. The Ag NP synthesis was carried out at different temperatures (e.g., 0 °C and room temperature) with different AgNO3/NaBH4 ratios (e.g., 1:1, 1:2, 2:1). A liquid−liquid separator equipped with a 1 μm pore PTFE membrane was inserted after the PFA coil reactor to effect continuous separation. The aqueous stream (retentate) containing Ag NPs was collected and analyzed. The heptane stream (permeate) was collected in a separate vial and recycled for other runs.

In-line Surface Enhanced Raman Spectroscopy Experiment

Raman spectra were acquired using a Horiba Jobin-Yvon confocal LabRam 300 spectrometer coupled with a microscope 776




and a detector. The detailed experimental procedure and equipment list are given in the student and instructor handouts in the Supporting Information. Ag NPs obtained during Workshop #1 and crystal violet (10−7 M in D.I. water in the presence of 0.2 M sodium chloride) were injected through a Tmixer into a 0.25 mL internal volume PFA microfluidic reactor (Figure 3) at 100 μL min−1 each. The outlet of the microfluidic

Figure 4. Illustration of segmented flow in a microfluidic PFA assembly.

Figure 3. Microfluidic setup for in-line SERS detection of crystal violet (CV) in the presence of Ag NPs in an aqueous medium.

can be correlated to both the average diameter and size distribution of the NPs. Normalized extinction spectra of some samples made of Ag NPs are plotted in Figure 5a. Typical high-quality spherical Ag

reactor was connected to a glass capillary where the SERS detection occurred directly using a 532 nm laser. The system was stabilized for 5 min, and the intensity of the spectrum was recorded every 15 s. A total running time of 150 s was selected, corresponding to 11 recorded spectra per sample. The intensity of the CV band located at 1621 cm−1 was measured on the spectrum after the background subtraction (LabSpec). Nanoparticle Characterization

The extinction spectra of Au and Ag NPs as well as CdSe and CdTe QDs were recorded by UV−vis spectroscopy.26,27 QDs were further characterized by fluorescence measurements. The structural characterization of Au/Ag NPs and QDs was performed using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), respectively.

Figure 5. (a) Normalized extinction spectra of three samples made of Ag NPs (sample 1 was obtained with a Ag/NaBH4 ratio of 1:2 at room temperature; samples 2 and 3 were obtained at 0 °C with Ag/NaBH4 ratios of 1:1 and 1:2, respectively). The lower inset is a TEM picture of sample 1. (a) Normalized extinction spectra of four samples made of Au NPs (samples 1−3 were obtained at 60 °C with Au/TSC ratios of 1:1, 1:2, and 2:2, respectively; sample 4 was obtained with a Au/TSC ratio of 1:1 at 85 °C). The lower inset is a TEM picture of sample 3. Solid lines correspond to the synthesized samples, while dashed lines correspond to theoretical LSPR. See the Supporting Information for experimental details.

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HAZARDS Minimum safety gear, including nitrile gloves, safety goggles, and a lab coat, must be worn during all experiments. Experiments under pressure and high temperature must be performed in a fume hood with the sash down. HAuCl4, NaBH4, and heptane can cause skin and eye damage and are toxic. Silver and gold solutions must be disposed in labeled containers. Cadmium salts are toxic, and their corresponding solutions must be stored in labeled containers and disposed through the university’s chemical waste program. Selenium and tellurium and their corresponding salts and solutions are toxic and must be stored in labeled containers. Chemical destruction of selenide and telluride solutions was performed by slowly adding concentrated nitric acid. The resulting solutions were disposed through the university’s chemical waste program. Contacting the local EH&S division for further assistance is strongly advised.

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NPs with sizes ranging from 10 to 60 nm exhibit a sharp Lorentzian-shaped LSPR that red-shifts from 395 to 430 nm as the NP size increases (Figure 5a, dashed lines). Here all of the samples had a similar large LSPR band with a nearly Lorentzian profile with a maximum located around λmax ≈ 400 nm. This observation suggests that the Ag NPs prepared under these microfluidic conditions are spherical with diameters ranging from 11 to 27 nm.13,32 Both the Ag/NaBH4 ratio and the temperature had a profound impact on the size of the Ag NPs: when the concentration of NaBH4 was doubled at 0 °C, the average size of the NPs decreased from 27 ± 8 nm to 11 ± 4 nm, and the corresponding UV spectra showed a blue shift of the LSPR band position (e.g., sample 3 in Figure 5a). When the reagent ratio was kept constant, the NP size decreased with increasing temperature, in agreement with the literature (e.g., Samples 1 and 3 in Figure 5a).33 The reduction of chloroauric acid was carried out with TSC at 60 and 85 °C (Figure 1). The influence of the reaction temperature and the Au/TSC ratio on the features of the Au NPs (size, size distribution, shape, and LSPR spectrum) can easily be studied as well. Normalized extinction spectra of some

RESULTS AND DISCUSSION

Workshop #1: Silver and Gold Nanoparticles

The concomitant injection of heptane and aqueous solutions into the microfluidic system led to the formation of a segmented flow regime (Figure 4) that prevented the formation of a silver layer on the wall of the microreactor. Reaction parameters such as temperature, residence time, and reagent ratio can be easily scouted under microfluidic conditions. Samples generated under various conditions are then analyzed by UV−vis spectroscopy, and the position of the localized surface plasmon resonance (LSPR) as well as the bandwidth 777




Au NPs samples are plotted in Figure 5b. The LSPR positions of these Au NPs samples were in the range of 528−561 nm (i.e., sizes ranging from 30 to 90 nm).34 As soon as the TSC concentration is doubled, the LSPR red-shifts from 541 to 547 nm (e.g., samples 1 and 2 in Figure 5b), while when both the citrate and gold concentrations are doubled, the LSPR redshifts from 541 to 561 nm (e.g., sample 3 in Figure 5b). However, when the Au/TSC ratio is kept constant and the temperature of the reaction is increased from 60 to 85 °C, the LSPR blue-shifts from 541 to 528 nm (e.g., sample 4 in Figure 5b). These results indicate that a higher reaction temperature reduces the size of Au NPs, in agreement with the literature.35 TEM analyses confirmed these results and showed a nearly perfect spherical shape for all of the samples (diameters ranging from 34 to 80 nm).

Figure 7. (top) Evolution of the fluorescence maximum wavelength and (bottom) calculated diameter of CdSe QDs synthesized in octadecene as functions of the residence time.

Workshop #2: Cadmium Selenide and Cadmium Telluride Quantum Dots

The growth kinetics of CdSe QDs can be illustrated by increasing the residence time in the microreactor: increasing the residence time from 15 s to 7 min 30 s leads to CdSe QDs solutions that turned from light yellow to orange, respectively (Figure 6a). A more quantitative correlation between the optical properties and the size of the QDs can be accessed through absorption and fluorescence spectroscopies (Figure 6b).

Similar trends were observed while using method B, yet the color depth of these samples increased markedly, which indicates that larger QDs are produced (4 to 6 nm for residence times ranging from 15 s to 8 min). This was later confirmed by HRTEM. CdTe QDs could also be prepared in the same microfluidic setup at higher temperature in water. Absorption and fluorescence measures on these samples nicely illustrate the influence of the QD composition on its spectral features (see the Supporting Information). Workshop #3: In-line Surface-Enhanced Raman Spectroscopy Experiment

SERS is at least 106 times more sensitive than classical Raman spectroscopy and has a great potential for in situ trace analysis in aqueous solution.37 The Ag NPs synthesized during Workshop #1 were utilized for the implementation of in-line SERS detection of trace amounts of CV in water (Figures 3 and 8).37 The SERS spectra obtained for CV in the absence and in

Figure 6. (a) Visible-light-illuminated (top) and UV light (395 nm)illuminated (bottom) CdSe QDs synthesized using method A. (b) Absorbance spectra (left) and fluorescence spectra under 405 nm illumination (right) of CdSe QDs as functions of the residence time.

Figure 8. Raman spectra obtained for aqueous CV (10−7 M) in the absence (red solid line) and in the presence (black solid line) of Ag NPs.

As the residence time increased, the CdSe QD absorption progressively increased, and the maximum absorption peak became more and more resolved. As illustrated in Figure 6b, both the absorption and fluorescence maxima red-shift with increasing residence time and thus with the expected increasing QD size.25 Using an empirical equation and the fluorescence band36 allowed the evolution of the QD diameter to be plotted as a function of the residence time (Figure 7 and the Supporting Information). This color change resulted from the increase in QD size with increasing residence time and constituted a simple and powerful visual demonstration of size-dependent properties.

the presence of Ag NPs are shown in Figure 8. The results were obtained from measurements on 10−7 M aqueous solutions of CV in the presence of 0.2 M sodium chloride. Figure 8 clearly illustrates the enhancement of the signal in the presence of Ag NPs by up to a factor of 104. The main limitation of these workshops is actually not related to the microfluidic assemblies or the analytical instrumentation but rather to the availability of precision syringe pumps. Alternatives such as HPLC pumps can be considered as well. The workshops are highly appreciated since 778


Journal of Chemical Education they combine the assembly of microfluidic devices and their utilization for the preparation of nanomaterials and successfully helped to convey the subject matter at hand through practice in the lab. In particular, students were fascinated by segmented flow regimes, membrane separation, the colorful illustration of QD growth as a function of the residence time, and the potential of SERS detection for trace analysis. This project is multidisciplinary in essence and helps students to connect different topics of their curriculum. Key concepts such as fluid dynamics at the microscale, Taylor (segmented) flow, membrane separation, and nanoparticle synthesis and characterization are taught. A wide range of skills are mobilized, ranging from dexterity to creativity, scientific rigor, and observation. The possibility to construct microfluidics assemblies from rather standard lab equipment demystifies microfluidics, makes it easily accessible, and most importantly stimulates students’ creativity. The participants have demonstrated not only an understanding of the fundamental concepts of microfluidics but also a genuine curiosity about its applicability to other areas of chemistry. These workshops also offer a unique platform for illustrating advanced analytical techniques.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00899. Instructor’s handout (PDF, DOCX) Student’s handout (PDF, DOCX)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Marjorie Lismont). *E-mail: [email protected] (Gauthier Eppe). *E-mail: [email protected] (Jean-Christophe M. Monbaliu). ORCID

Jean-Christophe M. Monbaliu: 0000-0001-6916-8846 Author Contributions ∥

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CONCLUSION This multidisciplinary project illustrates fundamental concepts of chemical processing in microfluidic devices and gives a unique opportunity to combine this emerging technology with the preparation and analysis of various inorganic nanostructures. Students learn the construction of microfluidic setups with affordable and widely accessible PFA capillaries and HPLC connectors. The tips and techniques students gather from this project can be easily applied to other branches of chemistry. Besides microfluidics, this integrated project also illustrates the most common techniques and tools for the analysis and characterization of inorganic nanoparticles. The last workshop emphasizes the coupling of the microfluidic preparation of Ag NPs with the analytical detection of trace amounts of CV with SERS.

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ACKNOWLEDGMENTS

The authors acknowledge financial support from the University of Liège (WG-13/03, J.-C.M.M.). The authors are also grateful to the upper undergraduate chemistry students who contributed to the improvement of these workshops over the last 3 years.

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N.E. and G.E.-A. contributed equally to this work.

Notes

The authors declare no competing financial interest. 779




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