Bubble gate for in-plane flow control

We introduce a miniature gate valve as a readily implementable strategy for actively controlling the flow of liquids on-chip, within a footprint of less than one square millimetre. Bubble gates provide for simple, consistent and scalable control of liquid flow in microchannel networks, are compatible with different bulk microfabrication processes and substrate materials, and require neither electrodes nor moving parts. A bubble gate consists of two microchannel sections: a liquid-filled channel and a gas channel that intercepts the liquid channel to form a T-junction. The open or closed state of a bubble gate is determined by selecting between two distinct gas pressure levels: the lower level corresponds to the "open" state while the higher level corresponds to the "closed" state. During closure, a gas bubble penetrates from the gas channel into the liquid, flanked by a column of equidistantly spaced micropillars on each side, until the flow of liquid is completely obstructed. We fabricated bubble gates using single-layer soft lithographic and bulk silicon micromachining procedures and evaluated their performance with a combination of theory and experimentation. We assessed the dynamic behaviour during more than 300 open-and-close cycles and report the operating pressure envelope for different bubble gate configurations and for the working fluids: de-ionized water, ethanol and a biological buffer. We obtained excellent agreement between the experimentally determined bubble gate operational envelope and a theoretical prediction based on static wetting behaviour. We report case studies that serve to illustrate the utility of bubble gates for liquid sampling in single and multi-layer microfluidic devices. Scalability of our strategy was demonstrated by simultaneously addressing 128 bubble gates.

A simple millifluidic benchtop reactor system for the high-throughput synthesis and functionalization of gold nanoparticles with different sizes and shapes

Despite the continuing interest in the applications of functionalized nanomaterials, the controlled and reproducible synthesis of many important functionalized nanoparticles (NPs) above the milligram scale continues to be a significant challenge. The synthesis of functionalized NPs in automated reactors provides a viable approach to circumvent some of the shortcomings of traditional nanomaterial batch syntheses, providing superior control over reagent addition, improved reproducibility, the opportunity to interface real-time product monitoring, and a viable high-throughput synthetic approach. Here, we demonstrate the construction and operation of a simple millifluidic reactor assembled entirely from commercially available components found in almost any chemical laboratory. This reactor facilitates the aqueous gram-scale synthesis of a variety of functionalized gold nanoparticles, including the synthesis of gold nanospheres with tightly controlled core diameters and gold nanorods with controlled aspect ratios between 1.5 and 4.0. The absolute dimensions (i.e., the transverse diameter) of gold nanorods synthesized within the reactor can also be tailored to produce different gold nanorod shapes, including "small" gold nanorods and gold nanocubes. In addition, we show that the reactor can interface with existing purification and monitoring techniques in order to enable the high-throughput functionalization/purification of gold nanorods and real-time monitoring of gold nanoparticle products for quality control. We anticipate that this millifluidic reactor will provide the blueprint for a versatile and portable approach to the gram-scale synthesis of monodisperse, hydrophilically functionalized metal NPs that can be realized in almost any chemistry research laboratory.

Fine control over the size of surfactant-polyelectrolyte nanoparticles by hydrodynamic flow focusing

Synthesis of surfactant-polyelectrolyte nanoparticles was carried out in a microfluidic device with a fine control over the size and the polydispersity. An anionic polysaccharide (sodium carboxymethylcellulose, CMC) solution was focused using a cationic surfactant (dodecyl trimethylammonium bromide, DTAB) solution in a microfluidic channel at selected ratios of flow rates and reagent concentrations. The methodology ensured a controlled mixing kinetics and a uniform distribution of charges at the mixing interface. The resulting nanoparticles exhibited remarkably well-defined and repeatable size distributions, with hydrodynamic diameters tunable from 50 up to 300 nm and polydispersity index around 0.1 in most cases. Microfluidic-assisted self-assembly may be an efficient way to produce well-controlled polyelectrolyte-based nanoparticles suitable for colloidal science as well as for gene delivery applications.

Novel process windows for enabling, accelerating, and uplifting flow chemistry

Novel Process Windows make use of process conditions that are far from conventional practices. This involves the use of high temperatures, high pressures, high concentrations (solvent-free), new chemical transformations, explosive conditions, and process simplification and integration to boost synthetic chemistry on both the laboratory and production scale. Such harsh reaction conditions can be safely reached in microstructured reactors due to their excellent transport intensification properties. This Review discusses the different routes towards Novel Process Windows and provides several examples for each route grouped into different classes of chemical and process-design intensification.

Segmented flow reactors for nanocrystal synthesis

In the past decade microreactors have emerged as a compelling technology for the highly controlled synthesis of colloidal nanocrystals, offering multiple advantages over conventional batch synthesis methods (including improved levels of control, reproducibility, and automation). Initial work in the field employed simple continuous phase reactors that manipulate miscible streams of a single reagent phase. Recently, however, there has been increasing interest in segmented flow reactors that use an immiscible fluid to divide the reagent phase into discrete slugs or droplets. Key advantages of segmented flow include the elimination of velocity dispersion (a significant cause of polydispersity) and greatly reduced susceptibility to reactor fouling. In this progress report we review the operation of segmented flow microreactors, their application to the controlled synthesis of nanocrystals, and some of the principal challenges that must be addressed before they can become a mainstream technology for the controlled production of nanomaterials.

Femtolitre chemistry assisted by microfluidic pen lithography

Chemical reactions at ultrasmall volumes are becoming increasingly necessary to study biological processes, to synthesize homogenous nanostructures and to perform high-throughput assays and combinatorial screening. Here we show that a femtolitre reaction can be realized on a surface by handling and mixing femtolitre volumes of reagents using a microfluidic stylus. This method, named microfluidic pen lithography, allows mixing reagents in isolated femtolitre droplets that can be used as reactors to conduct independent reactions and crystallization processes. This strategy overcomes the high-throughput limitations of vesicles and micelles and obviates the usually costly step of fabricating microdevices and wells. We anticipate that this process enables performing distinct reactions (acid-base, enzymatic recognition and metal-organic framework synthesis), creating multiplexed nanoscale metal-organic framework arrays, and screening combinatorial reactions to evaluate the crystallization of novel peptide-based materials.

Challenges and opportunities in the advancement of nanomedicines

Nanomedicine-based approaches to cancer treatment face several challenges that differ from those encountered by conventional medicines during clinical development. A systematic exploration of these issues has led us to identify the following needs and opportunities for further development: (1) robust and general methods for the accurate characterization of nanoparticle size, shape, and composition; (2) scalable approaches for producing nanomedicines with optimized bioavailability and excretion profiles; (3) particle engineering for maintaining low levels of nonspecific cytotoxicity and sufficient stability during storage; (4) optimization of surface chemistries for maximum targeted delivery and minimum nonspecific adsorption; (5) practical methods for quantifying ligand density and distributions on multivalent nanocarriers; and (6) the design of multifunctional nanomedicines for novel combination therapies with supportable levels of bioaccumulation.

Microwave-assisted synthesis of CdSe quantum dots: can the electromagnetic field influence the formation and quality of the resulting nanocrystals?

Microwave-assisted syntheses of colloidal nanocrystals (NCs), in particular CdSe quantum dots (QDs), have gained considerable attention due to unique opportunities provided by microwave dielectric heating. The extensive use of microwave heating and the frequently suggested specific microwave effects, however, pose questions about the role of the electromagnetic field in both the formation and quality of the produced QDs. In this work a one-pot protocol for the tunable synthesis of monodisperse colloidal CdSe NCs using microwave dielectric heating under carefully controlled conditions is introduced. CdSe QDs are fabricated using selenium dioxide as a selenium precursor, 1-octadecene as a solvent and reducing agent, cadmium alkyl carboxylates or alkyl phosphonates as cadmium sources, 1,2-hexadecanediol to stabilize the cadmium complex and oleic acid to stabilize the resulting CdSe QDs. Utilizing the possibilities of microwave heating technology in combination with accurate online temperature control the influence of different reaction parameters such as reaction temperature, ramp and hold times, and the timing and duration of oleic acid addition have been carefully investigated. Optimum results were obtained by performing the reaction at 240 °C applying a 5 min ramp time, 2 min hold time before oleic acid addition, 90 s for oleic acid addition, and a 5 min hold time after oleic acid addition (8.5 min overall holding at 240 °C). By using different cadmium complexes in the microwave protocol CdSe QDs with a narrow size distribution can be obtained in different sizes ranging from 0.5-4 nm by simply changing the cadmium source. The QDs were characterized by TEM, HRTEM, UV-Vis, and photoluminescence methods and the size distribution was monitored by SAXS. Control experiments involving conventional conductive heating under otherwise identical conditions ensuring the same heating and cooling profiles, stirring rates, and reactor geometries demonstrate that the electromagnetic field has no influence on the generated CdSe QDs. The resulting CdSe NCs prepared using either conductive or microwave dielectric heating exhibited the same primary crystallite size, shape, quantum yield and size distribution regardless of the heating mode.

Microfluidic one-step fabrication of radiopaque alginate microgels with in situ synthesized barium sulfate nanoparticles

In this work, we report a new strategy to fabricate monodispersed radiopaque alginate (Ba-alginate) microgels by a one-step microfluidic method. Alginate droplets containing sulfate ions are first formed by a flow focusing microfluidic setup. These alginate droplets are subsequently solidified by barium ions in a collection bath. During the solidification process, excessive barium ions in the collection bath also react with sulfate ions in the alginate droplet, resulting in barium sulfate (BaSO(4)) nanoparticles in situ synthesized (acting as radiopaque imaging agents) within the Ba-alginate microgels. Scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX) illustrate that 800 nm BaSO(4) nanoparticles are uniformly distributed inside the 30 μm Ba-alginate microgels, with 62 wt% of elemental barium (Ba). In addition, X-ray diffraction (XRD) measurements indicate that the BaSO(4) nanoparticles consist of 10 nm in situ synthesized BaSO(4) crystallites. The alginate microgels act as a soft and porous template to prevent the precipitation and aggregation of BaSO(4) nanoparticles. The Ba-alginate microgels are also visible under X-ray radiation. The facile route to fabricate alginate microgels as radiopaque embolic materials is of particular importance for endovascular embolization and localized diagnostic imaging applications. Similar approaches can also be adopted for synthesizing other inorganic nanoparticles in microgels.

Microfluidic technologies for accelerating the clinical translation of nanoparticles

Using nanoparticles for therapy and imaging holds tremendous promise for the treatment of major diseases such as cancer. However, their translation into the clinic has been slow because it remains difficult to produce nanoparticles that are consistent 'batch-to-batch', and in sufficient quantities for clinical research. Moreover, platforms for rapid screening of nanoparticles are still lacking. Recent microfluidic technologies can tackle some of these issues, and offer a way to accelerate the clinical translation of nanoparticles. In this Progress Article, we highlight the advances in microfluidic systems that can synthesize libraries of nanoparticles in a well-controlled, reproducible and high-throughput manner. We also discuss the use of microfluidics for rapidly evaluating nanoparticles in vitro under microenvironments that mimic the in vivo conditions. Furthermore, we highlight some systems that can manipulate small organisms, which could be used for evaluating the in vivo toxicity of nanoparticles or for drug screening. We conclude with a critical assessment of the near- and long-term impact of microfluidics in the field of nanomedicine.

Mechanism of co-nanoprecipitation of organic actives and block copolymers in a microfluidic environment

Microreactors have been shown to be a powerful tool for the production of nanoparticles (NPs); however, there is still a lack of understanding of the role that the microfluidic environment plays in directing the nanoprecipitation process. Here we investigate the mechanism of nanoprecipitation of block copolymer stabilized organic NPs using a microfluidic-based reactor in combination with computational fluid dynamics (CFD) modelling of the microfluidic implementation. The latter also accounts for the complex interplay between molecular and hydrodynamic phenomena during the nanoprecipitation process, in order to understand the hydrodynamics and its influence on the NP formation process. It is demonstrated that the competitive reactions result in the formation of two types of NPs, i.e., either with or without loading organic actives. The obtained results are interpreted by taking into consideration a new parameter representing the mismatching between the aggregations of the polymers and actives, which plays a decisive role in determining the size and polydispersity of the prepared hybrid NPs. These results expand the current understanding of the co-nanoprecipitation mechanism of active and block copolymer stabilizer, and on the role exerted by the microfluidic environment, giving information that could be translated to the emerging fields of microfluidic formation of NPs and nanomedicine.

Controlled synthesis of fluorescent silica nanoparticles inside microfluidic droplets

We study the droplet-based synthesis of fluorescent silica nanoparticles (50-350 nm size) in a microfluidic chip. Fluorescein-isothiocyanate (FITC) dye is first chemically linked to aminopropyl triethoxysilane (APTES) in ethanol and this reaction product is subsequently mixed with tetraethyl orthosilicate (TEOS) to yield a fluorescent silicon alkoxide precursor solution. The latter reacts with an aqueous ethanol-ammonia hydrolysing mixture inside droplets, forming fluorescent silica nanoparticles. The droplets are obtained by pinching-off side-by-side flowing streams of alkoxide solution/hydrolysing mixture on a microfluidic chip using a Fluorinert oil continuous phase flow. Synthesis in droplets leads to a faster reaction and allows drastically improved nanoparticle size uniformity (down to 3% relative standard deviation for 350 nm size particles) when compared to conventional bulk synthesis methods, thanks to the precise control of reagent concentrations and reaction times offered by the microfluidic format. Incorporating FITC inside silica nanoparticles using our method leads to reduced dye leakage and increases the dye's stability, as evidenced by a reduced photochemical bleaching compared to a pure FITC solution.

Highly controlled synthesis of nanometric gold particles by citrate reduction using the short mixing, heating and quenching times achievable in a microfluidic device

Homodispersed 1.8 nm gold nanoparticles were obtained reproducibly in high yields using the classical Turkevich protocol at a high concentration in a continuous flow capillary reactor. The microfluidic reactor made from commercially available items permitted short mixing, heating and quenching times which are the key parameters of this synthesis.

Digital microfluidics: a versatile tool for applications in chemistry, biology and medicine

Digital microfluidics (DMF) has recently emerged as a popular technology for a wide range of applications. In DMF, nanoliter to microliter droplets containing samples and reagents can be manipulated to carry out a range of discrete fluidic operations simply by applying a series of electrical potentials to an array of patterned electrodes coated with a hydrophobic insulator. DMF is distinct from microchannel-based fluidics as it allows for precise control over multiple reagent phases (liquids and solids) in heterogeneous systems with no need for complex networks of connections, microvalves, or pumps. In this review, we discuss the most recent developments in this technology with particular attention to the potential benefits and outstanding challenges for applications in chemistry, biology, and medicine.

One-step continuous synthesis of biocompatible gold nanorods for optical coherence tomography

We present a novel one-step flow process to synthesize biocompatible gold nanorods with tunable absorption and biocompatible surface ligands. Photothermal optical coherence tomography (OCT) of human breast tissue is successfully demonstrated using tailored gold nanorods designed to have strong absorption in the near-infrared range.

Size-controlled flow synthesis of gold nanoparticles using a segmented flow microfluidic platform

Segmented flow is often used in the synthesis of nanomaterials to achieve narrow particle size distribution. The narrowness of the distribution is commonly attributed to the reduced dispersion associated with segmented flows. On the basis of the analysis of flow fields and the resulting particle size distribution, we demonstrate that it is the slip velocity between the two fluids and internal mixing in the continuous-phase slugs that govern the nature of the particle size distribution. The reduction in the axial dispersion has less impact on particle growth and hence on the particle size distribution. Synthesis of gold nanoparticles from HAuCl(4) with rapid reduction by NaBH(4) serves as a model system. Rapid reduction yields gold nuclei, which grow by agglomeration, and it is controlled by the interaction of the nuclei with local flow. Thus, the difference in the physical properties of the two phases and the inlet flow rates ultimately control the particle growth. Hence, a careful choice of continuous and dispersed phases is necessary to control the nanoparticle size and size distribution.

Controlling bubbles using bubbles--microfluidic synthesis of ultra-small gold nanocrystals with gas-evolving reducing agents

Microfluidic wet-chemical synthesis of nanoparticles is a growing area of research in chemical microfluidics, enabling the development of continuous manufacturing processes that overcome the drawbacks of conventional batch-based synthesis methods. The synthesis of ultra-small (<5 nm) metallic nanocrystals is an interesting area with many applications in diverse fields, but is typically very challenging to accomplish in a microfluidics-based system due to the use of a strong gas-evolving reducing agent, aqueous sodium borohydride (NaBH(4)), which causes uncontrolled out-gassing and bubble formation, flow disruption and ultimately reactor failure. Here we present a simple method, rooted in the concepts of multiphase mass transfer that completely overcomes this challenge-we simply inject a stream of inert gas bubbles into our channels that essentially capture the evolving gas from the reactive aqueous solution, thereby preventing aqueous dissolved gas concentration from reaching the solubility threshold for bubble nucleation. We present a simple model for coupled mass transfer and chemical reaction that adequately captures device behaviour. We demonstrate the applicability of our method by synthesizing ultra-small gold nanocrystals (<5 nm); the quality of nanocrystals thus synthesized is further demonstrated by their use in an off-chip synthesis of high-quality gold nanorods. This is a general approach that can be extended to a variety of metallic nanomaterials.

Targeted polymeric therapeutic nanoparticles: design, development and clinical translation

Polymeric materials have been used in a range of pharmaceutical and biotechnology products for more than 40 years. These materials have evolved from their earlier use as biodegradable products such as resorbable sutures, orthopaedic implants, macroscale and microscale drug delivery systems such as microparticles and wafers used as controlled drug release depots, to multifunctional nanoparticles (NPs) capable of targeting, and controlled release of therapeutic and diagnostic agents. These newer generations of targeted and controlled release polymeric NPs are now engineered to navigate the complex in vivo environment, and incorporate functionalities for achieving target specificity, control of drug concentration and exposure kinetics at the tissue, cell, and subcellular levels. Indeed this optimization of drug pharmacology as aided by careful design of multifunctional NPs can lead to improved drug safety and efficacy, and may be complimentary to drug enhancements that are traditionally achieved by medicinal chemistry. In this regard, polymeric NPs have the potential to result in a highly differentiated new class of therapeutics, distinct from the original active drugs used in their composition, and distinct from first generation NPs that largely facilitated drug formulation. A greater flexibility in the design of drug molecules themselves may also be facilitated following their incorporation into NPs, as drug properties (solubility, metabolism, plasma binding, biodistribution, target tissue accumulation) will no longer be constrained to the same extent by drug chemical composition, but also become in-part the function of the physicochemical properties of the NP. The combination of optimally designed drugs with optimally engineered polymeric NPs opens up the possibility of improved clinical outcomes that may not be achievable with the administration of drugs in their conventional form. In this critical review, we aim to provide insights into the design and development of targeted polymeric NPs and to highlight the challenges associated with the engineering of this novel class of therapeutics, including considerations of NP design optimization, development and biophysicochemical properties. Additionally, we highlight some recent examples from the literature, which demonstrate current trends and novel concepts in both the design and utility of targeted polymeric NPs (444 references).

Engineering of micro- and nanostructured surfaces with anisotropic geometries and properties

Widespread approaches to fabricate surfaces with robust micro- and nanostructured topographies have been stimulated by opportunities to enhance interface performance by combining physical and chemical effects. In particular, arrays of asymmetric surface features, such as arrays of grooves, inclined pillars, and helical protrusions, have been shown to impart unique anisotropy in properties including wetting, adhesion, thermal and/or electrical conductivity, optical activity, and capability to direct cell growth. These properties are of wide interest for applications including energy conversion, microelectronics, chemical and biological sensing, and bioengineering. However, fabrication of asymmetric surface features often pushes the limits of traditional etching and deposition techniques, making it challenging to produce the desired surfaces in a scalable and cost-effective manner. We review and classify approaches to fabricate arrays of asymmetric 2D and 3D surface features, in polymers, metals, and ceramics. Analytical and empirical relationships among geometries, materials, and surface properties are discussed, especially in the context of the applications mentioned above. Further, opportunities for new fabrication methods that combine lithography with principles of self-assembly are identified, aiming to establish design principles for fabrication of arbitrary 3D surface textures over large areas.

Developing a millifluidic platform for the synthesis of ultrasmall nanoclusters: ultrasmall copper nanoclusters as a case study

The future of lab-on-a-chip devices for the synthesis of nanomaterials hinges on the successful development of high-throughput methods with better control over their size. While significant effort in this direction mainly focuses on developing "difficult to fabricate" complex microfluidic reactors, scant attention has been paid to the "easy to fabricate" and simple millifluidic systems that could provide the required control as well as high throughput. By utilizing numerical simulation of fluids within the millifluidic space at different flow rates, the results presented here show velocity profiles and residence time distributions similar to the case of microfluidics. By significantly reducing the residence time and residence time distribution, a continuous flow synthesis of ultrasmall copper nanoclusters (UCNCs) with exceptional colloidal stability is achieved. In-situ synchrotron-radiation-based X-ray absorption spectroscopy (XAS) reveal that the as-prepared clusters are about 1 nm, which is further supported by transmission electron microscopy and UV-vis spectroscopy studies. The clusters reported here are the smallest ever produced using a lab-on-a-chip platform. When supported on silica, they are found to efficiently catalyze C-H oxidation reactions, hitherto unknown to be catalyzed by Cu. This work suggests that a millifluidic platform can be an inexpensive, versatile, easy-to-use, and powerful tool for nanoparticle synthesis in general, and more specifically for ultrasmall nanoclusters (UNCs).

Locally enhanced concentration and detection of oligonucleotides in a plug-based microfluidic device

We propose a novel technique that allows oligonucleotides with specific end-modification within a plug in a plug-based microfluidic device to undergo a locally enhanced concentration at the rear of the plug as the plug moves downstream. DNA was enriched and detected in situ upon exploiting a combined effect underlain by an entropic force induced through fluid shear (i.e. a hydrodynamic-repellent effect) and the interfacial adsorption (aqueous/oil interface) attributed to affinity. Flow fields within a plug were visualized quantitatively using micro-particle image velocimetry (micro-PIV); the distribution of the fluid shear strain rate explains how the hydrodynamic-repellent effect engenders a dumbbell-like region with an increased concentration of DNA. The concentration of FAM (6-carboxy-fluorescein)-labeled DNA (FC-DNA) and of TAMRA (tetramethyl-6-carboxyrhodamine)-labeled DNA (TC-DNA), respectively, and the hybridization of probe DNA (modified with FAM) with target DNA (modified with TAMRA) were investigated in devices; a confocal fluorescence microscope (CFM) was utilized to monitor the processes and to resolve the corresponding 2D patterns and 3D reconstruction of the DNA distribution in a plug. TC-DNA, but not FC-DNA, concentrating within a plug was affected by the combined effect so as to achieve a concentration factor (C(r)) twice that of FC-DNA because of the lipophilicity of TAMRA. Using fluorescence resonance-energy transfer (FRET), we characterized the hybridization of the DNA in a plug; the detection limit of a system, improved by virtue of the proposed technique (the locally enhanced concentration), for DNA detection was estimated to be 20-50 nM. This technique enables DNA to concentrate locally in a nL-pL free-solution plug, the locally enhanced concentration to profit the hybridization efficiency and the detection of DNA, prospectively serving as a versatile means to accomplish a rapid DNA detection in a small volume for a Lab-on-a-Chip (LOC) system.

A practical guide for the fabrication of microfluidic devices using glass and silicon

This paper describes the main protocols that are used for fabricating microfluidic devices from glass and silicon. Methods for micropatterning glass and silicon are surveyed, and their limitations are discussed. Bonding methods that can be used for joining these materials are summarized and key process parameters are indicated. The paper also outlines techniques for forming electrical connections between microfluidic devices and external circuits. A framework is proposed for the synthesis of a complete glass/silicon device fabrication flow.

Continuous-flow catalytic asymmetric hydrogenations: Reaction optimization using FTIR inline analysis

The asymmetric organocatalytic hydrogenation of benzoxazines, quinolines, quinoxalines and 3H-indoles in continuous-flow microreactors has been developed. Reaction monitoring was achieved by using an inline ReactIR flow cell, which allows fast and convenient optimization of reaction parameters. The reductions proceeded well, and the desired products were isolated in high yields and with excellent enantioselectivities.

A ceramic microreactor for the synthesis of water soluble CdS and CdS/ZnS nanocrystals with on-line optical characterization

In this paper, a computer controlled microreactor to synthesize water soluble CdS and CdS/ZnS nanocrystals with in situ monitoring of the reaction progress is developed. It is based on ceramic tapes and the Low-Temperature Co-fired Ceramics technology (LTCC). As well the microsystem set-up, the microreactor fluidic design has also been thoroughly optimized. The final device is based on a hydrodynamic focusing of the reagents followed by a three-dimensional micromixer. This generates monodispersed and stable CdS and core-shell CdS/ZnS nanocrystals of 4.5 and 4.2 nm, respectively, with reproducible optical properties in terms of fluorescence emission wavelengths, bandwidth, and quantum yields, which is a key requirement for their future analytical applications. The synthetic process is also controlled in real time with the integration of an optical detection system for absorbance and fluorescence measurements based on commercial miniaturized optical components. This makes possible the efficient managing of the hydrodynamic variables to obtain the desired colloidal suspension. As a result, a simple, economic, robust and portable microsystem for the well controlled synthesis of CdS and CdS/ZnS nanocrystals is presented. Moreover, the reaction takes place in aqueous medium, thus allowing the direct modular integration of this microreactor in specific analytical microsystems, which require the use of such quantum dots as labels.