(618n) Versatile Preparation of Monodisperse Carbon Microspheres In a Simple Microfluidic Device

^{Versatile Preparation of Monodisperse Carbon
Microspheres in a Simple Microfluidic
Device}

Yichang Pan,
Minhua Ju, Ya Liu,
Chongqing
Wang, Lixiong Zhang,* and Nanping Xu

State Key Laboratory
of Materials-Oriented Chemical Engineering and
College of
Chemistry and Chemical Engineering,

Nanjing University of Technology,
Nanjing
210009, P. R. China. Fax: 86 25 8317 2263; Tel: 86 25 8317 2265; E-mail: lixiongzhang@yahoo.com

Microspheres
composed of polymeric or inorganic materials are of great interests because of
their potential application as catalyst supports, carriers for cosmetics,
drugs, protein and DNA delivery, electrodes, sensors and so on. They are
prepared by various methods such as emulsion/micelles/vesicle polymerization,
spray drying, template methods, and self¨Ctemplate method. Recently, a new
technique was developed to synthesize uniform-sized hollow microspheres such as
nylon, biopolymer, organosilicon and TiO₂ using microfluidic
emulsion technique via interfacial polymerization at the surface of the
microdroplet.

In this report, we
fabricated hollow and solid poly(furfuryl alcohol) (PFA) microspheres with
diameters in the range from tens to several hundreds of micrometers by the
phase transfer of FA and polymerization in the aqueous droplets using
microfluidic devices. We
fed an oil solution containing FA, fatty acid methyl ester, and a surfactant
(span 80) and an sulfuric acid aqueous solution separately into the microfluidic
devices to form an aqueous solution microdroplets dispersed in the oil phase.
FA in the oil phase would diffuse into the droplets, and consequently start to
polymerize at the catalysis of sulfuric acid. Thus, a layer of PFA would be
formed at the surface of aqueous droplet, resulting in formation of PFA
microcapsules containing aqueous solution. The PFA microcapsules in-turn could
be transformed to microporous carbon hollow microspheres by carbonization.
Solid PFA microspheres could also be prepared by prolonging the reaction time.

We first prepared
PFA microspheres with diameters of several hundreds of micrometers in a
microfluidic device assembled by inserting a syringe needle perpendicularly
into a polyvinyl chloride (PVC) tube. By changing the flow rates of oil and
aqueous solutions, residence time of aqueous microdroplets in the fluidic
device, FA concentration in oil solution and curing temperature, the particle
size and wall thickness of PFA hollow microspheres can be easily adjusted. In
addition, a magnetic precursor could be simply incorporated in PFA
microspheres, leading to formation of magnetic carbon hollow microspheres. By
introducing silica sols in the aqueous phase, PFA-SiO₂ composite
microspheres were obtained, which were then converted into carbon-silica
composite microspheres after carbonization and finally the carbon microspheres
after HF etching. The resultant carbon microspheres were mesoporous because
in-situ polymerized silica gel networks served as hard template, and their mesoporosity
depended on the flow rate of the oil phase and the residence time of the
microdroplets in the device during the preparation of PFA-SiO₂
microspheres. The highest mesoporous volume and mesoporosity of carbon
microspheres reached 1.06 cm³.g^-1
and 82 %, respectively, and the mesopores were in the range of 2~20 nm.
Mesoporous silica microspheres embeded with submicron-sized cavities could also
be prepared by burning-off the carbon in the PFA- SiO₂ microspheres.