High temperature continuous flow synthesis of CdSe/CdS/ZnS, CdS/ZnS, and CdSeS/ZnS nanocrystals

Flow Chemistry: A Sustainable Voyage Through the Chemical Universe en Route to Smart Manufacturing

Microfluidic devices and systems have entered many areas of chemical engineering, and the rate of their adoption is only increasing. As we approach and adapt to the critical global challenges we face in the near future, it is important to consider the capabilities of flow chemistry and its applications in next-generation technologies for sustainability, energy production, and tailor-made specialty chemicals. We present the introduction of microfluidics into the fundamental unit operations of chemical engineering. We discuss the traits and advantages of microfluidic approaches to different reactive systems, both well-established and emerging, with a focus on the integration of modular microfluidic devices into high-efficiency experimental platforms for accelerated process optimization and intensified continuous manufacturing. Finally, we discuss the current state and new horizons in self-driven experimentation in flow chemistry for both intelligent exploration through the chemical universe and distributed manufacturing. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Continuous-flow syntheses of alloy nanoparticles

Alloy nanoparticles (NPs), including core-shell, segregated and solid-solution types, show a variety of attractive properties such as catalytic and optical properties and are used in a wide range of applications. Precise control and good reproducibility in the syntheses of alloy NPs are highly demanded because these properties are tunable by controlling alloy structures, compositions, particle sizes, and so on. To improve the efficiency and reproducibility of their syntheses, continuous-flow syntheses with various types of reactors have recently been developed instead of the current mainstream approach, batch syntheses. In this review, we focus on the continuous-flow syntheses of alloy NPs and first overview the flow syntheses of NPs, especially of alloy NPs. Subsequently, the details of flow reactors and their chemistry to synthesize core-shell, segregated, solid-solution types of alloy NPs, and high-entropy alloy NPs are introduced. Finally, the challenges and future perspectives in this field are discussed.

Flow synthesis of photocatalytic semiconductor-metal hybrid nanocrystals

Semiconductor-metal hybrid nanostructures are promising materials for photocatalytic applications, providing high efficiencies compared to their composing counterparts. So far, the synthesis of such hybrid nanoparticles was limited to batch reactors, achieving tunability while demonstrating how each of the nanocrystals' characteristics affects photocatalytic performances. Yet, new methodologies should be established to increase the synthetic yield while maintaining high control over the resulting structures. Herein, scalable advanced flow techniques are introduced, yielding ZnSe-metal hybrid nanoparticles either in a thermal growth or photo-induced growth regime. Firstly, thermal gold growth in the flow reactor is achieved with good control over the metal tip size and the nanoparticle morphology. We address the dependence of the reaction on temperature, the precursor to nanorod molar ratios, and additional parameters. Additionally, light-induced growth by the flow reactor is demonstrated for platinum clusters. The quality of the resulting hybrids is directly demonstrated by their functionality in photocatalytic hydrogen generation by water reduction, displaying enhanced activity compared to bare ZnSe nanorods. The fairly straightforward adaptation of such powerful flow-reaction techniques to scale-up photocatalytic hybrid nanoparticle syntheses takes them one step forwards towards the realization of their potential in real-life application scenarios.

Microfluidic based nanoparticle synthesis and their potential applications

A lot of substantial innovation in advancement of microfluidic field in recent years to produce nanoparticle reveals a number of distinctive characteristics for instance compactness, controllability, fineness in process, and stability along with minimal reaction amount. Recently, a prompt development, as well as realization in production of nanoparticles in microfluidic environs having dimension of micro to nanometers and constituents extending from metals, semiconductors to polymers, has been made. Microfluidics technology integrates fluid mechanics for production of nanoparticles having exclusive with homogenous sizes, shapes, and morphology, which are utilized in several bioapplications such as biosciences, drug delivery, healthcare, including food engineering. Nanoparticles are usually well-known for having fine and rough morphology because of their small dimensions including exceptional physical, biological, chemical, and optical properties. Though the orthodox procedures need huge instruments, costly autoclaves, use extra power, extraordinary heat loss, as well as take surplus time for synthesis. Additionally, this is fascinating in order to systematize, assimilate, in addition, to reduce traditional tools onto one platform to produce micro and nanoparticles. The synthesis of nanoparticles by microfluidics permits fast handling besides better efficacy of method utilizing the smallest components for process. Herein, we will focus on synthesis of nanoparticles by means of microfluidic devices intended for different bioapplications. This article is protected by copyright. All rights reserved.

Polymer-capped CdSe/ZnS quantum dots for the sensitive detection of Cu2+ and Hg2+ and the quenching mechanism

In this work, poly(styrene-co-maleic anhydride)-capped CdSe/ZnS quantum dots (QDs) aminolyzed with ethanolamine are proposed as fluorescent probes for the detection of Cu2+ and Hg2+, and two different quenching mechanisms are discussed in detail. The coordination abilities of the surface polymer of CdSe/ZnS QDs and two metal ions are calculated by density functional theory (DFT). The photoinduced electron transfer from excited QDs to Cu2+ unoccupied orbitals is enhanced due to the coordination between Cu2+ and the surface polymer of QDs. The electron transfer consumes non-radiative energy and performs fluorescence quenching. For Hg2+, the formation of HgS and the slight aggregation of polymer-coated CdSe/ZnS QDs lead to fluorescence quenching. The probe is sensitive to both Cu2+ and Hg2+, and the response can be detected within 1 min without adjusting the pH. With the addition of a masking agent, Cu2+ and Hg2+ can be exclusively detected in coexistence with another ion. For Cu2+, a linear relation in the concentration ranging from 0.02 to 0.7 μM was found between the relative fluorescence intensity (F0/F) and the concentration of Cu2+; the limit of detection (S/N = 3) is 6.94 nM. For Hg2+, a linear relation ranging from 0.1 to 1.4 μM was found between ln(F0/F) and the concentration of Hg2+; the limit of detection is 20.58 nM.

Microfluidic synthesis of quantum dots and their applications in bio-sensing and bio-imaging

Bio-sensing and bio-imaging of organisms or molecules can provide key information for the study of physiological processes or the diagnosis of diseases. Quantum dots (QDs) stand out to be promising optical detectors because of their excellent optical properties such as high brightness, stability, and multiplexing ability. Diverse approaches have been developed to generate QDs, while microfluidic technology is one promising path for their industrial production. In fact, microfluidic devices provide a controllable, rapid and effective route to produce high-quality QDs, while serving as an effective in situ platform to understand the synthetic mechanism or optimize reaction parameters for QD production. In this review, the recent research progress in microfluidic synthesis and bio-detection applications of QDs is discussed. The definitions of different QDs are first introduced, and the advances in microfluidic-based fabrication of quantum dots are summarized with a focus on perovskite QDs and carbon QDs. In addition, QD-based bio-sensing and bio-imaging technologies for organisms of different scales are described in detail. Finally, perspectives for future development of microfluidic synthesis and applications of QDs are presented.

Engineering Brightness Matched Indium Phosphide Quantum Dots

The size-dependent optoelectronic properties of semiconductor nanocrystals quantum dots (QDs) are hugely beneficial for color tunability but induce an inherent relative PL brightness mismatch in QDs emitting different colors, as larger emitters absorb more incident photons than smaller particles. Here, we examine the effect of core composition, shell composition, and shell thickness on optical properties including high energy absorption, quantum yield (QY), and the relative brightness of InP/ZnS and InP/ZnSe core/shell and InP/ZnSe/ZnS core/shell/shell QDs at different excitation wavelengths. Our analysis reveals that the presence of an intermediate ZnSe shell changes the wavelength of enhanced absorption onset and leads to highly excitation wavelength dependent QYs. Switching from commercial CdSe/ZnS to InP/ZnS reduces the brightness-mismatch between green and red emitters from 33- to 5-fold. Incorporating a 4-monolayer thick optically absorbing ZnSe shell into the QD heterostructure and heating the QDs in a solution of zinc oleate and trioctylphosphine produces InP/ZnSe/ZnS QDs that are ~10-fold brighter than their InP/ZnS counterparts. In contrast to CdSe/CdS/ZnS core/shell/shell QDs, which only photoluminesce at red wavelengths with thicker CdS shells due to their Quasi-Type II bandstructure, Type I InP/ZnSe/ZnS QDs are uniquely suited to creating a rainbow of visible-emitting, brightness matched emitters. By tailoring the thickness of the intermediate ZnSe shell, heavy metal-free, brightness-matched green and red emitters are produced. This study highlights the ability to overcome the inherent brightness mismatch seen in QDs through concerted materials design of heterostructured core/shell InP-based QDs.

A high-temperature continuous stirred-tank reactor cascade for the multistep synthesis of InP/ZnS quantum dots

Multistep and continuous production of core–shell InP/ZnS semiconductor nanocrystals in a high-temperature and miniature continuous stirred-tank reactor cascade.

Optical and Electronic Properties of Colloidal CdSe Quantum Rings

Luminescent colloidal CdSe nanorings are a recently developed type of semiconductor structure that have attracted interest due to the potential for rich physics arising from their nontrivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single-particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (∼10%) compared to platelets (∼30%) is attributed to an enhanced coupling between (i) excitons and CdSe LO-phonons at 200 cm^-1 and (ii) negatively charged selenium-rich traps, which give nanorings a high surface charge (∼-50 mV). Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but also a result of traps introduced by puncturing the platelet center.

Synthesis and Surface Engineering of Inorganic Nanomaterials Based on Microfluidic Technology

The controlled synthesis and surface engineering of inorganic nanomaterials hold great promise for the design of functional nanoparticles for a variety of applications, such as drug delivery, bioimaging, biosensing, and catalysis. However, owing to the inadequate and unstable mass/heat transfer, conventional bulk synthesis methods often result in the poor uniformity of nanoparticles, in terms of microstructure, morphology, and physicochemical properties. Microfluidic technologies with advantageous features, such as precise fluid control and rapid microscale mixing, have gathered the widespread attention of the research community for the fabrication and engineering of nanomaterials, which effectively overcome the aforementioned shortcomings of conventional bench methods. This review summarizes the latest research progress in the microfluidic fabrication of different types of inorganic nanomaterials, including silica, metal, metal oxides, metal organic frameworks, and quantum dots. In addition, the surface modification strategies of nonporous and porous inorganic nanoparticles based on microfluidic method are also introduced. We also provide the readers with an insight on the red blocks and prospects of microfluidic approaches, for designing the next generation of inorganic nanomaterials.

Inorganic nanoparticle synthesis in flow reactors – applications and future directions

The use of flow technologies for obtaining nanoparticles can play an important role in the development of ecological and sustainable processes for obtaining inorganic nanomaterials, and the continuous methods are part of the Flow Chemistry trend.

Continuous production of iron oxide nanoparticles via fast and economical high temperature synthesis

A continuous, fast and economical high temperature synthesis of iron oxide nanoparticles was developed and compared to a conventional batch synthesis in terms of production costs.

On-demand Milifluidic Synthesis of Quantum Dots in Digital Droplet Reactors

Colloidal quantum dots (QDs) offer dramatic potential due to their size-dependent optical properties. Lack of facile synthesis methods for precise and reproducible size and composition, however, present an extant barrier to their widespread use. Here we report the use of droplet microfluidics for the simple and highly reproducible synthesis of cadmium sulfide (CdS) and cadmium selenide (CdSe) QDs without the use of harsh solvents and in ambient conditions. Our approach uses a liquid-liquid barrier between two immiscible liquids to generate a digital droplet reactor. This reaction droplet is easily controlled and manipulated and offers enhanced mixing when coupled to a helical mixer, resulting in a significant reduction in size distribution compared to benchtop procedures. Furthermore, QD characteristics have modeled and predicted based on the parameters of the microfluidic device. We believe this method overcomes the current manufacturing challenges with synthesizing nanostructures, which is required for the next generation of nanosensors.

Continuous Flow Methods of Fabricating Catalytically Active Metal Nanoparticles

One of the obstacles preventing the commercialization of colloidal nanoparticle catalysts is the difficulty in fabricating these materials at scale while maintaining a high level of control over their resulting morphologies, and ultimately, their properties. Translation of batch-scale solution nanoparticle syntheses to continuous flow reactors has been identified as one method to address the scaling issue. The superior heat and mass transport afforded by the high surface-area-to-volume ratios of micro- and millifluidic channels allows for high control over reaction conditions and oftentimes results in decreased reaction times, higher yields, and/or more monodisperse size distributions compared to an analogous batch reaction. Furthermore, continuous flow reactors are automatable and have environmental health and safety benefits, making them practical for commercialization. Herein, a discussion of continuous flow methods, reactor design, and potential challenges is presented. A thorough account of the implementation of these technologies for the fabrication of catalytically active metal nanoparticles is reviewed for hydrogenation, electrocatalysis, and oxidation reactions.

Controllable synthesis of nanocrystals in droplet reactors

In recent years, a broad range of nanocrystals have been synthesized in droplet-based microfluidic reactors which provide obvious advantages, such as accurate manipulation, better reproducibility and reliable automation. In this review, we initially introduce general concepts of droplet reactors followed by discussions of their main functional regions including droplet generation, mixing of reactants, reaction controlling, in situ monitoring, and reaction quenching. Subsequently, the enhanced mass and heat transport properties are discussed. Next, we focus on research frontiers including sequential multistep synthesis, intelligent synthesis, reliable scale-up synthesis, and interfacial synthesis. Finally, we end with an outlook on droplet reactors, especially highlighting some aspects such as large-scale production, the integrated process of synthesis and post-synthetic treatments, automated droplet reactors with in situ monitoring and optimizing algorithms, and rapidly developing strategies for interfacial synthesis.

Millifluidic synthesis of cadmium sulfide nanoparticles and their application in bioimaging

In this work, a miniature fluidic synthesis platform utilizing millimeter dimension channels yielding highly reproducible batch synthesis of luminescent cadmium sulfide (CdS) quantum dots and nanocrystals is demonstrated.