Synthesis of carbohydrate-functionalized quantum dots in microreactors.

Large quantities of monodisperse semiconductor nanocrystals, quantum dots (QDs), are needed for applications in electronics and the life sciences. For biological applications, the surface of QDs is often functionalized with carboxylic acids for the attachment of proteins or directly with carbohydrates. Traditional batch processes are of limited utility for the production of QDs on a larger scale owing to limited temperature control and lack of homogeneous mixing. Continuous-flow microreactors provide precise control over reaction conditions, including temperature, and the production time is independent of the process scale. The high surface-to-volume ratio of the microreactor channels enables precise temperature control as well as efficient mixing, allowing for the preparation of QDs with narrow size distribution. QDs have been prepared using microfabricated gas–liquid and liquid–liquid flow reactors. The preparation of surface-functionalized QDs under mild reaction conditions in the liquid phase remains challenging. Ideally, a continuous process would serve to both produce the quantum dots and to functionalize them. Herein we present a single-phase microfluidic system for the synthesis of highly luminescent, surface-functionalized CdSe and CdTe nanoparticles. In contrast to batch processes, which require temperatures of 250–300 8C, temperatures of 160 8C are sufficient in the flow process. Both the formation of the zinc sulfide shell and the functionalization of the nanoparticles with carboxy groups and carbohydrates were perfomed in a continuous-flow system (Figure 1). Differentsized quantum dots were obtained by simply varying the reaction time in the flow reactor. High reaction temperatures usually result in fast nucleation, and large nanocrystals are quickly obtained. At low temperatures, the size of the nanocrystals and the concentration of the unreacted precursors in the mixture can be balanced. Thus, continuous nucleation is suppressed, the residence time distribution (RTD) is narrowed, and homogeneous QD fractions are obtained by varying the reaction time. The homogenous reaction mixture and slow nucleation results in a mild process for the production of QDs using microreactors. CdSe and CdTe nanoparticles with different emission maxima were prepared by injection of a 1:1 mixture of Cd precursor and Se or Te precursor. The Cd precursor was prepared by the addition of oleic acid and oleylamine to a solution of cadmium oxide dissolved in lauric acid at 150 8C. The Se and Te precursors were prepared by dissolving elemental selenium or tellerium powder in tri-n-octylphosphine (TOP) in a Syrris microreactor. Reaction times ranged from 3 to 30 minutes. The CdSe and CdTe cores were purified by precipitation from methanol/chloroform/n-hexane and dried under vacuum. The average size distribution of each sample was calculated from the absorbance spectra (see Figure 1 in the Supporting Information). The optical properties of the QDs show a time-dependent bathochromic shift in the band-edge emission and enhanced intensity. The photoluminescence peaks of CdSe QDs are sharp, with fwhm (full width at half maximum) values of the band-edge luminescence between 40 and 50 nm (Figure 2), which indicates the narrow size distribution of the QDs. However, after 30 minutes of reaction time the fwhm increased from 40 to 90 nm, and a decrease in quantum yield indicated that saturated nucleation occurred after 20– Figure 1. Microreactor setup for the continuous-flow synthesis of functionalized QDs (OA: oleic acid; TOP: tri-n-octylphosphine).