Mechanistic Insights and Controlled Synthesis of Radioluminescent

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Mechanistic insights and controlled synthesis of radioluminescent ZnSe quantum dots using a microfluidic reactor Eder Jose Guidelli, Ioannis Lignos, Jason Jungwan Yoo, Marcella Lusardi, Moungi G. Bawendi, Oswaldo Baffa, and Klavs F. Jensen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03587 • Publication Date (Web): 12 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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Chemistry of Materials

Mechanistic insights and controlled synthesis of radioluminescent ZnSe quantum dots using a microfluidic reactor Eder Jose Guidelli,†‡ Ioannis Lignos,†§ Jason Jungwan Yoo,§ Marcella Lusardi,† Moungi G. Bawendi,§ Oswaldo Baffa,‡ and Klavs F. Jensen†* †

Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cam‐ bridge, MA 02139, U.S.A

‡

Departamento de Física – FFCLRP, Universidade de São Paulo

§

Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A

ABSTRACT: We describe the controlled colloidal synthesis and characterization of ZnSe quantum dots using a continuous flow microfluidic reactor. A systematic investigation of the synthetic route reveals a possible two stage pathway for ZnSe nanocrystal formation. The first stage corresponds to the formation of zinc selenide nuclei at low temperatures (160oC), followed by the growth of ZnSe nanocrystals at higher temperatures (340oC). The quantum dots exhibit sharp exciton ab‐ sorption, with tunable emission spectra between 370 and 430 nm. The photoluminescence of ZnSe nanocrystals is charac‐ terized by narrow emission linewidths of 14‐21 nm. For the first time, we report luminescent emission from ZnSe nanocrys‐ tals upon x‐ray excitation, revealing that radioluminescence emission is associated to confined excitons, and that the radi‐ oluminescence intensity is a linear function of the fluence/dose‐rate of x‐rays. The precise control of the synthesis of parti‐ cles with uniform sizes and excellent optical properties associated with the microfluidic synthesis opens a new avenue for the controlled production of heavy‐metal‐free luminescent and radioluminescent nanocrystals in flow.

■ INTRODUCTION To date, semiconductor nanocrystals have received great attention as luminescent materials for a wide range of ap‐ plications, such as light emitting diodes, flat panel displays, and lasers.1–6 Microfluidic technology has become a useful tool to efficiently develop reproducible synthetic method‐ ologies for the production of engineered nanocrystals,7–12 with controlled optical, electronic, and catalytic proper‐ ties.8,10 In addition, flow synthesis offers several advantages compared to conventional synthetic methods (flask‐based methodologies) for the preparation of nanostructures in‐ cluding better temperature control, rapid mixing, product reproducibility, in‐situ characterization techniques for fast parametric screening of reaction conditions, optimization algorithms, and better surface engineering. 8,10,13 Over the last two decades, there have been numerous stud‐ ies for the synthetic control of quantum dots using micro‐ fluidic reactors.14–20 However, most of the continuous‐flow methods developed for nanocrystal production are de‐ voted to the synthesis of quantum dots based on toxic ele‐ ments, including cadmium and lead.14,21–23 Such materials have been severely restricted in several countries due to their potential toxicity.24,25 It is therefore necessary to de‐ velop new approaches for the reproducible and controlla‐ ble synthesis of heavy‐metal‐free luminescent nanocrys‐ tals.26–29

Indium phosphide quantum dots are potential substitutes of Cd‐ and Pb‐based nanocrystals due to their lower tox‐ icity.25,30 However, their stability in water and oxygen re‐ mains an issue.25,31–33 ZnSe quantum dots represent a robust alternative as one of the least potentially toxic nanomateri‐ als with high photoluminescence (PL) quantum yields.34 In addition, the PL spectra from ZnSe based quantum dots can be tuned from the ultraviolet (UV) to the blue region of the electromagnetic spectrum (370‐450 nm). Moreover, they can be also be tuned into the red region by impurity doping and/or surface modification.35–46 Thus far, there is only one work reporting the production of ZnSe quantum dots using microfluidic reactors.44 The lack of a well‐estab‐ lished microfluidic synthesis for the production of ZnSe quantum dots stems from the low solubility of most of the Zn precursors as well as the low reactivity between Zn and Se. Therefore, developing a reproducible synthetic meth‐ odology for the formation of low‐toxicity nanocrystals us‐ ing microfluidic platforms remains a challenge. Besides investigating the absorbance and PL properties of heavy‐metal‐free semiconductor nanocrystals, some appli‐ cations in biomedicine also require investigation of the ra‐ dioluminescence (RL) emission.47–50 RL is the light emitted by the investigated materials when exposed to ionizing ra‐ diation, like x‐rays.47,51 Particularly, the ionization caused by x‐rays produces electron‐hole pairs in the insulator and semiconducting materials, that can recombine leading to light emission in the UV‐visible region47,51. This lumines‐ cent process is also called scintillation, x‐ray luminescence,

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or x‐ray excited optical luminescence (xeol).47,52 RL emit‐ ting nanocrystals have recently been employed for the de‐ velopment of new cancer therapies combining radiation and photodynamic therapy (x‐ray activated photodynamic therapy). 47,49,53,54 In these treatments, the energy harvested by the RL emitting nanocrystals from the radiation therapy x‐ray beam is transferred to photosensitizers and produces reactive oxygen species. 47 In addition, because the inten‐ sity of RL is proportional to the dose rate of x‐rays,51 RL can also be used for assessing doses of ionizing radiation in in‐ dustrial and medical applications55. In some materials, the electrons and holes created by the ionizing radiation can be trapped in the gap,56–58 or even lose their energies by collision with defects in the crystal structure and/or by non‐radiative transitions.47,59,60 It is therefore evident that RL requires trap‐free high‐quality crystals, which is even more challenging when the investi‐ gated materials have dimensions on the nanoscale. In this work, we have developed a synthetic methodology for the formation of low‐toxicity and RL emitting ZnSe nanocrys‐ tals for biomedical applications using a continuous‐flow microfluidic reactor. Quantum dots with a high degree of crystallinity and tunable sizes are obtained by controlling the reaction time (30s to 10 min). Absorption and PL spec‐ troscopy are performed and RL properties of ZnSe quan‐ tum dots are also demonstrated here for the first time. The quantum dots are luminescent upon UV and X‐ray excita‐ tion with tunable emission bands (380‐430 nm) and nar‐ row emission linewidths (14‐21 nm).

■ EXPERIMENTAL SECTION Microreactor Fabrication: A tubular microfluidic reactor was fabricated for the formation of ZnSe quantum dots (Fig.1). The reactor consisted of three parts: 1) a syringe pump to precisely control the flow rates and reaction times, 2) the tubular microfluidic reactor; and 3) the sam‐ ple collector. Compared to silicon chip reactors, stainless‐ steel tube reactors are cheaper and easier to assemble with a wide choice of dimensions and lengths and can resist high temperatures and pressures.33 A 500 µm stainless steel tube (Mcmaster‐Carr) was wrapped around a cylindrical aluminum rod (5 cm diameter) with a heating cartridge in‐ serted in the center for heating, and a thermocouple placed close to the stainless‐steel tubes for accurate temperature control. A fiberglass and wool insulation layer covers the entire microfluidic reactor. For some synthetic parameters, a backpressure regulator was also implemented in the sys‐ tem. Backpressure Regulator

ZnSe Nuclei

Syringe Pump Tubular Microreactor/ Heating Zone

Sample Collector

Figure 1. Schematic of the microfluidic reactor used for the synthesis of ZnSe quantum dots.

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Materials: 1‐octadecene (ODE, 90%), trioctylphosphine (TOP, 97%), oleic acid (OLA, 90%), diethylzinc (1 M solu‐ tion in hexane) and anhydrous solvents of acetone, isopro‐ panol, hexane, and toluene were obtained from Aldrich. Selenium powder (99.999%) and stearic acid were pur‐ chased from Alfa Aesar. Flow synthesis of ZnSe cores: In a typical synthesis, pre‐ cursors were prepared in‐situ and ZnSe cores were synthe‐ sized according to the following procedure: 11 mL 1‐octade‐ cene and 0.1 mmol stearic acid were added into a three‐ neck flask and degassed at 110oC for 30 min. Then, under argon flow, the reaction mixture was cooled to 80oC and 0.1 mmol diethyl zinc was injected and stirred for 1 minute, followed by a 3‐minute degassing period. Thereafter, 0.8 mmol Se (2.2 M TOP‐Se solution) was injected and the re‐ action mixture was heated to 160oC (10oC/min heating rate) for approximately 10‐40 min (depending on the initial con‐ centration of precursors) under argon flow, until the ex‐ tinction at 300 nm reached 0.4 a.u. (for a dissolution of 100 µL of precursor solution in 1 ml of toluene in a quartz cu‐ vette with 1 cm optical length). Subsequently, the precur‐ sor solution was collected with a glass syringe and injected in a tubular microfluidic reactor with varying reaction times (30 s – 10 min) and temperatures. The syringe was kept at 80oC during the entire experiment to avoid precip‐ itation of the stearic acid. Characterization: Absorption spectra were recorded us‐ ing a Cary 5000 UV‐Vis‐NIR infrared spectrometer, and PL spectra were recorded using a Fluoromax‐3 spectrofluo‐ rometer (Horiba Jobin Jvon). Quantum yields of the syn‐ thesized nanocrystals were determined by the relative method using 9,10‐diphenyl‐anthracene diluted in toluene (100% quantum yield) as a reference.61 High resolution transmission electron microscopy (HRTEM) images were captured using a JEOL 2010F at 200kV. X‐ray diffraction (XRD) patterns were recorded using a PANalytical XPert instrument, in the range of 25–70°, to verify the crystalline nature and structure of the nanocrystals. RL measurements were performed under X‐Ray irradiation from an X‐ray tube (Isovolt Titan E‐160M‐2 GE), with 2 mm aluminum and 0.8 mm beryllium filtration, at 160 kVp and varying tube current (2‐10 mA). The emitted light was collected and analyzed using a fiber‐optic spectrometer. The X‐ray dose rate at the sample position is 35 Gy/min.

■RESULTS AND DISCUSSION Typically, the synthesis of high‐quality ZnSe quantum dots employs zinc stearate as the source of zinc and as a ligand. However, the solubility of zinc stearate in most of the sol‐ vents is low, hindering its use as a precursor for the micro‐ fluidic synthesis. To overcome this limitation, we proposed the use of diethyl zinc, which is soluble in non‐polar sol‐ vents, and stearic acid as ligand. In addition, we deter‐ mined that a precursor solution containing 10 mM of stea‐ ric acid in ODE was suitable for a flow experiment. This


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allows for altering zinc concentration and stearic acid:Zn:Se molar ratios, avoiding reactor clogging. Initially, all precursors (0.1 mmol stearic acid, 0.1 mmol Di‐ ethyl Zinc and 0.8 mmol TOP‐Se in 1‐octadecene) were mixed in a three‐neck flask and subsequently injected into the reactor at 310oC without any preheat treatment. How‐ ever, we observed changes in the PL emission as a function of the time the precursors were kept inside the syringe (in‐ itially at 120oC) (Figure S1). Absorbance measurements of the samples collected at different temperatures (80oC‐ 240oC) revealed that the optical density of the solution in‐ creased with temperature (Figure 2a). The absence of an excitonic peak for temperatures lower than 160oC sug‐ gested an initial stage of nucleation. For temperatures above 200oC, the appearance of an excitonic peak in the range of 350‐375 nm indicated the growth of ZnSe quantum dots. For a more detailed investigation of the initial nucle‐ ation stage (0.1 mol.L‐1) lead to a viscous fluid at temperatures above 80oC, and a solid solution at temperatures below 80oC, due to the high melting point of stearic acid. There‐ fore, we examined the effect of viscosity of the reaction me‐ dium on the optical properties of the synthesized nano‐ crystals by fixing the stearic Acid/Zn/Se molar ratio (1:1:8) while altering the overall concentration of precursors and ligand between 0.05 – 0.15 mmol. Preheating time was 70 min, 40 min, and 20 min, for stearic acid concentrations of 0.05 mmol, 0.1 mmol, and 0.15 mmol, respectively to obtain the same concentration of nuclei before the injection of the precursor solution into the microfluidic reactor (the lower the concentration of precursors the longer the time re‐ quired for nucleation). The recorded PL spectra (Figure 5a) reveal that 0.05 mmol of stearic acid leads to faster growth and quantum dots with trap‐free emission are produced within 30 s residence time. The nanocrystals presented well‐defined first and second excitonic absorption peaks and PL linewidth (14 nm) even narrower than the obtained for higher acid concentration (18 nm – 10min reaction time). To our knowledge, this is so far the narrowest PL linewidth of ZnSe quantum dots produced in microfluidic

reactors. Longer residence times (4 and 6 min) led to de‐ fined excitonic absorption peaks but very broad (>100 nm) PL emissions (Figure 5b), suggesting the presence of sur‐ face trap states probably due to the lower concentration of stearic acid/zinc stearate, or even total consumption of precursors. For samples produced with 0.1 mmol stearic acid, single PL emission is only obtained for residence times longer than 2 min (Figure 5d)), with a PL linewidth of 21 nm. Further increase of the acid (0.15 mmol stearic acid) led to even broader emissions at short residence times (Figure 5f), sug‐ gesting that high concentration of the ligand decreases the diffusion of the precursors/growth species towards the nu‐ clei surfaces, thereby requiring longer residence time for the growth of the nanocrystals. This is in agreement with the proposed ZnSe nucleation and growth mechanism (Figure 4). To further investigate the effect of stoichiometry on parti‐ cle growth, we conducted various reactions under different stearic acid:Zn:Se molar ratios. The preheating time (time to achieve a 0.4 a.u. absorbance at 300 nm) decreased upon increasing zinc concentration, suggesting a faster nuclea‐ tion process. Figure 6 indicates that upon increasing the Zn



concentration (from 0.05 to 0.8 mmol) PL linewidths in‐ creases from 21 to 38 nm. The broadening of the PL emis‐ sion may be due to the higher concentration of diethyl zinc in the solution injected in the microfluidic reactor, with both zinc stearate and diethyl zinc participating in particle growth. Furthermore, due to its high reactivity, the re‐ maining/excess diethyl zinc may have led to simultaneous nucleation and growth in the microfluidic reactor at 340oC, leading to broader particle size distribution and, conse‐ quently, broader PL linewidths. We note that the luminescence intensity and quantum yield of ZnSe cores (0.1 stearic acid:0.1 Zn:0.8 Se – 340oC) decreases upon increasing the reaction time (Figure 7a‐b), with its highest values at 6% (reaction time = 30 s). The high luminescence efficiency from quantum dots relies on the strong overlap of the electron and hole wave functions in the confined structure, but the role of surface ligands is also important. Therefore, we speculate that the lower quantum yield of ZnSe quantum dots produced with reac‐ tion times above 2 min is probably caused by the decreased concentration of ligands during the growth for longer re‐ action times. Besides characterization of the excitonic absorption, PL and quantum‐yields of the ZnSe cores, we also investigated their RL properties. Typically, ZnSe single crystals, pow‐ ders and films are used for radiation detection,80–82 but , to our knowledge, the RL properties of ZnSe nanocrystals have not been reported previously. For RL measurements, ZnSe cores prepared with 40 min preheating and 2 min re‐ action time were selected due to the optimal spectral prop‐ erties. RL spectra of ZnSe cores exposed

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The preheating time is 40 min and the temperature was fixed at 340oC. The ratio of stearic acid, zinc and selenium is kept as 1:1:8 (0.1 mmol stearic acid, 0.1 mmol Zn, 0.8 mmol Se).

to x‐rays reveal that ZnSe nanocrystals are also radiolumi‐ nescent materials for radiation detection (Figure 8a). Fur‐ thermore, the RL intensity increases linearly as a function of the current applied in the x‐ray tube (Figure 8b), i.e., RL intensity is linearly proportional to the fluence/dose‐rate of x‐rays, which is highly desirable.49 The RL emission band shifts according to the nanocrystal size (Figure 8c), evi‐ dencing that this emission is associated to confined exci‐ tons. Although the quantum‐yield of the as‐synthesized cores is relatively low, RL is detected. The formation of ZnSe/ZnS core/shell nanocrystals could increase the quan‐ tum‐yield and enhance further the RL emission.

Figure 8. (a) RL emission spectra from ZnSe cores in ODE upon different currents applied to the X‐ray tube. (b) RL in‐ tensity as a function of the tube current reveals a linear behav‐ ior. (c) RL emission band shifts according to the nanocrystal size.

■CONCLUSION

Figure 7. (a) A series of quantum yield measurements from the ZnSe cores as a function of reaction time. (b) Colloidal so‐ lutions of various ZnSe quantum dots in toluene with increas‐ ing residence time (left to right) under UV‐lamp irradiation.

In summary, we have developed a new microfluidic proto‐ col for the synthesis of ZnSe quantum dots, with high re‐ producibility and precise control of particle sizes. A sys‐ tematic investigation of the synthetic route revealed a two‐ stage pathway for the ZnSe nanocrystals nucleation and growth. In the first stage, zinc stearate is produced by the reaction of diethyl zinc and stearic acid. Thereafter, reac‐ tion of remaining diethyl zinc with TOP‐Se at 160oC leads to the formation of ZnSe nuclei. Injection of these ZnSe nu‐ clei into the microfluidic reactor at 340oC promotes growth of the nuclei into larger quantum dots. The quantum dots had sharp excitonic absorption peaks and narrow PL lin‐ ewidths. Besides optical characterization, RL properties of the ZnSe cores were evaluated for the first time. Similar to PL, the RL emission is also associated to confined excitons, and its intensity varies linearly as a function of the flu‐ ence/dose‐rate of x‐rays. We anticipate that employing a multistage microfluidic synthesis for the controlled for‐ mation of ZnSe/ZnS quantum dots with higher PL quan‐ tum yields and RL efficiencies will further promote their implementation in biomedical applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website and includes additional emission


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and absorption spectra, and TEM images of ZnSe quantum dots.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all au‐ thors. All authors have given approval to the final version of the manuscript.

Funding Sources This work was supported by the funding agencies FAPESP, CNPq, CAPES, and the National Science Foundation (NSF). I. Lignos was supported by a Swiss National Foundation Grant P2EZP2_172127.

ACKNOWLEDGMENT The authors thank E. de Paula, C. R. da Silva, and L. Rocha for technical assistance.

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