Research Article Stop Flow Lithography Synthesis and...

Research ArticleStop Flow Lithography Synthesis and Characterization ofStructured Microparticles

David Baah1 Tobias Donnell2 Sesha Srinivasan3 and Tamara Floyd-Smith12

1Materials Science amp Engineering Department Tuskegee University Tuskegee AL 36088 USA2Chemical Engineering Department Tuskegee University Tuskegee AL 36088 USA3College of Innovation and Technology Florida Polytechnic University Lakeland FL 33805 USA

Correspondence should be addressed to Tamara Floyd-Smith tfloydmytutuskegeeedu

Received 8 August 2014 Revised 10 November 2014 Accepted 11 November 2014 Published 31 December 2014

Academic Editor Theodorian Borca-Tasciuc

Copyright copy 2014 David Baah et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

In this study the synthesis of nonspherical composite particles of poly(ethylene glycol) diacrylate (PEG-DA)SiO2and PEG-

DAAl2O3with single ormultiple vias and the corresponding inorganic particles of SiO

2andAl

2O3synthesized using the Stop Flow

Lithography (SFL) method is reported Precursor suspensions of PEG-DA 2-hydroxy-2-methylpropiophenone and SiO2or Al2O3

nanoparticles were preparedThe precursor suspension flows through amicrofluidic devicemounted on an uprightmicroscope andis polymerized in an automated process A patterned photomask with transparent geometric features masks UV light to synthesizethe particles Composite particles with vias were synthesized and corresponding inorganic SiO

2and Al

2O3particles were obtained

through polymer burn-off and sintering of the composites The synthesis of porous inorganic particles of SiO2and Al

2O3with vias

and overall dimensions in the range of sim35ndash90 120583m was achieved BET specific surface area measurements for single via inorganicparticles were 56ndash69m2g for SiO

2particles and 73ndash81m2g for Al

2O3particles Surface areas as high as 114m2g were measured

for multivia cubic SiO2particlesThe findings suggest that with optimization the particles should have applications in areas where

high surface area is important such as catalysis and sieving

1 Introduction

Organic and inorganic micro- and nanoparticles and theircomposites with tailored morphologies exhibit unique struc-tural or shape dependent phenomena For these particlescontrol of shape and morphology are desirable for improvedapplication performance For example nonspherical particlesare promising as drug delivery vehicles due to the dramaticincrease in surface area-to-volume ratio (as a result ofdeparture from spherical shape to nonspherical shape) andimproved surface interaction (due to larger surface area)leading to increased cellular uptake into target organs [12] Consequently efforts to develop new methods (bothldquobottom-uprdquo and ldquotop-downrdquo) to synthesize particles withunique structural features have intensified Recent reviews ofthe literature highlight developments toward shape controlof micro- and nanoparticles with focus on scientifically andtechnologically important particles [3 4] Burda et al (2005)

explained that with the bottom-up approach particle forma-tion begins with nucleation and homogenous nucleation isgoverned by the driving forces of thermodynamics [5] Forspherical particles this driving force is expressed by the freeenergy change (Δ119866) of particle formation expressed in

Δ119866 (119903) = minus

4

119881

120587119903

3119896

119861119879 In (119878) + 41205871199032120574 (1)

where 119881 is the molecular volume of the precipitated species119903 is the radius of the nuclei 119896

119861is the Boltzmann constant 119878 is

the saturation ratio and 120574 is the surface energy per unit sur-face area [5] Even though it is the contention of the authorsthat particle growth follows the path of minimum energywhich largely favors the formation of spherical particles theyposited that particle formation is kinetically driven and thatanymetastable shape can be arrested by changing the reactionconditions Consequently surfactant molecules and differentconcentrations of monomers as capping agents as well as

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2014 Article ID 142929 9 pageshttpdxdoiorg1011552014142929

2 Journal of Nanomaterials

varying temperatures and pH of precursor solutions are usedas shape size and morphological control parameters follow-ing nucleation and during particle growth [5ndash9] Conditionssuch as slowing down the rate of polycondensation reactionin the sol gel synthesis of SiO

2nanoparticles yield smaller

sized particles whereas larger sized particles are obtained byincreasing the ammonia concentration [10] Other authorshave discussed the use of emulsion droplets hard and softtemplates sacrificial templates and template free approachesfor the synthesis of particles with unconventional structuresand their applications in batteries biomedicine catalysis andsensing [11ndash13] Additionally the independent or combinedapplications of the principles of Oswald ripening and Kirk-endall effects have been used to achieve the synthesis ofgenerally spherical and pseudospherical particles with hollowcavities for applications in gas adsorption and energy storage[14ndash22] Like hollow particles the introduction of small viasor orderedmacropores is a novelmeans of creating structureswith increased surface area [23]

Given the potential of nonspherical particles newapproaches have been developed to provide additional meth-ods to fill the need to synthesize them While microflu-idics technology is diverse in its usage including evalua-tion of nanobiomaterials and controlled release [24 25] itsapplication in particle design and synthesis is unparalleledRecent articles have catalogued specific microfluidic plat-forms particle formation precursors and conditions whichproducemyriad particles with uniquemorphological featuresand production rates [26ndash28] For example the stop flowlithography (SFL) technique used in this study producesparticles at a rate of 02 ghr and depends on the free radicalpolymerization of the precursor onto which UV light isprojected using a microscope SFL is limited to extrusionof two-dimensional particles and therefore is an excellentmethod for synthesis of nonspherical particles for the currentand potential applications that benefit from nonsphericalparticle shapes SFL controls the particle axial dimensionand the cross-sectional area using the microchannel depthand objective lens magnification respectively The versatilityof this method extends to the ability to synthesize encodedparticles for microRNA detection [29] with dimensions assmall as 800 nm realized [30] In previous work by the authorand others the SFL technique has been used to demonstratethe synthesis of nonspherical organic particles and theircomposites [31ndash33] as well as metal and metal oxide particles[34ndash36] However niche application requirements of metaloxide particles demand particles with ordered porosity whichpossess important physical properties such as high surfacearea favorable to heterogeneous catalysis supports sievesand adsorbents [37 38]

In this paper we report the synthesis of two-dimen-sionally extrudedmicroparticles with single andmultiple viasand different cross-sectional shapes using the SFL methodThe particles reported in this paper consist of high fidelityand solvent free PEG-DASiO

2and PEG-DAAl

2O3com-

posite particles and their corresponding inorganic particlesobtained through binder burn-out In this work microstruc-tured particles are those in which vias are introducedwith theaim of imparting additional surface area The corresponding

particle nanostructure results from the inorganic nanopar-ticles used to synthesize the microparticles This paper issignificant because it extends our work on the synthesis ofsolid metal oxide particles using SFL to microstructuredparticles with potential applications in the areas of catalysissupports sieves and adsorbents Also to our knowledge itprovides the first detailed characterization of the nanostruc-ture of these types of particles SiO

2and Al

2O3are common

catalyst support materials and were chosen because SiO2has

a low melting point and Al2O3was simultaneously being

investigated for abrasives applications However in theorythis method is applicable to anymetal oxide or other materialthat can be acquired in nanoparticle form and has a meltingpoint higher than 600∘C the temperature above which allpolymer binder is removed

2 Experimental

21 Materials Precursor suspensions are prepared using acommercial SiO

2dispersion in water (Ludox AS-40 or Ludox

CL-X colloidal silica) poly(ethylene glycol) diacrylate 2-hydroxy-2-methylpropiophenone hydrogen peroxide andascorbic acid all supplied by Sigma Aldrich as well as analuminum oxide nanodispersion (Al

2O3 20 wt 30 nm)

supplied by US Research Nanomaterials Inc The parti-cle synthesis was conducted using geometrically patternedphotomasks and in microfluidic devices whose fabricationmethods were described in a preceding work [31 39]

22 Particle Synthesis and Collection Figure 1 is a schematicillustration of the particle synthesis workstation Patternedphotomasks mask the UV light from a source This pho-tomask is inserted in the field stop of an upright micro-scope (Olympus BX51WF Microscope) and changed whennecessary to change the imposed cross-sectional shapes ofthe particles The UV light is focused onto the precursorflow stream using a 10x or 20x microscope objective lens Inaddition to light focusing the objective controls the particlecross-sectional size with the particle size decreasing as theobjective magnification increasesThe axial dimension of theparticles is controlled by the microfluidic channel depth In atypical operation a three-way solenoid valve regulates the air-pressure-driven precursor solution supply to themicrofluidicdevice mounted on the stage of a microscope The precursoris prepared bymixing aliquots of the commercial suspensionsof the inorganic nanoparticles with PEG-DA to constitutesuspensions containing 10wt Al

2O3and 20wt SiO

2in

30wt PEG-DA The goal was to use the highest possibleconcentration of nanoparticles in the suspension but theconcentrationwas limited by excessive fluid resistance to flow(high viscosity) caused by the particle loading as well as lightscattering and consequent inadequate binder crosslinking inspite of index matching The source of precursor supply isa 10mL syringe with the tip connected to the microfluidicdevice through ethyl vinyl acetate microbore tubing of 00410158401015840ID (WU-06492-04 Cole Parmer)The light source is a Lumen200 (Prior Scientific) equipped with a 200 watt metal arcbulb connected to a Shutter Driver (VCM-D1 Uniblitz)whose opening and closure are synchronized with those of

Journal of Nanomaterials 3

UV source

Patternedmask

Microfluidicdevice

Shutter

Microstructured particles

Particle outlet

Gas flowSolenoid valve

Compressed air supply

Shutter driver

10mL syringe(sample reservoir)

Figure 1 Schematic of the stop flow lithography (SFL) setup consisting of a UV light source a microfluidic device mounted on an uprightmicroscope and a prepolymer supply reservoir mounted on a clamp stand and connected to a solenoid valve to regulate sample flow to thedevice and particle ejection from the device

the solenoid valve to control the introduction of fresh pre-cursor solution as well as expelling synthesized particles A365 nm bright-line filter (Max Lamp Mercury Line Filter-Hg01-365-25 Semrock) is positioned in the light path to selectthe appropriate wavelength of 365 nm for the photoactiva-tion Once the shutter is opened the light comes throughto the sample and a free radical polymerization is triggeredwhich lasts for the period the light is on Typically withthis method particles are synthesized in 75ndash350ms exposuretimes in a fully automated process [36 39 40]

Particle separation is by gravity settling The process isdescribed in an earlier report [31] Further wash reagents(10 volhydrogen peroxide and 15wtvol ascorbic acid) areused for final washing to maintain particle fidelity Once thecomposite particles are collected washed free fromunreactedprecursor and stabilized they are sintered using a 1500∘CCompact Muffle Furnace (KSL-1500X-S) equipped with aprogrammable controller For the composite SiO

2particles

the furnace is ramped from room temperature to 600∘Cat 5∘Cmin to first remove moisture and then burn off thepolymer binder The temperature is held at 600∘C for 30minto ensure complete removal of binder and then rampedagain at sim45∘Cmin to 1150∘C and held for 8 hours Finallycontrolled cooling is conducted at sim3∘Cmin to 100∘C and thefurnace shuts off while the particles continue to cool to roomtemperature The process described produces consolidatedceramic particles Sintering of PEG-DAAl

2O3composite

particles follows a similar profile except that the maximumtemperature is 1450∘C near the limit of the equipment withat least a two-hour hold time

23 Characterization The zeta potential of the particles insuspension was measured using a Zeta Potential Analyzer(ZetaPlus BIC) First aliquots of the mother liquor arecentrifuged at 5000 rpm for 10mins and the supernatant wascollected A few drops of the mother liquor are added to

Table 1 Zeta potential of SiO2 and Al2O3 nanoparticles

SiO2 Al2O3

Solvent LudoxWater (119899 = 10) Isopropanol (119899 = 10)Concentration ( wt) 40 20Particle size (nm) 15 30Zeta potential (mV) minus209 plusmn 39 360 plusmn 26

the supernatant mixed thoroughly and used as sample Priorto sample measurement a zeta potential reference material(BI-ZR3) is analyzed to check the instrument performanceand reliability

Scanning electron microscopic (SEM) images of thesampleswere collected to show theirmicrostructural featuresFirst approximately 5mg of the dried aliquot sample is placedon a conductive carbon tape attached to the top of a 13mmradius aluminum stub and placed in an EMS 550X SputterCoating machine to coat the sample surface in a thin film ofgold Subsequently the sample is imaged using a JOEL JSM5800 or ZEISS EVO 50VP scanning electron microscope

The surface area and pore size distribution analyses arecarried out using Quantachrome Instrumentsrsquo Autosorb-iQBET surface area analysis Approximately 05 g of sample isloaded in a 6mm tube with a filler rod and outgassed for atleast 3 hrs at 300∘C after which a 7-point nitrogen physisorp-tion isotherm is performed at the liquid nitrogen temperatureof 77K The BET (Brunauer-Emmett-Teller) surface area ofthe samples is determined using ASiQwin II data analysissoftware

3 Results and Discussion

Table 1 shows results from the zeta potential characterizationof the precursor metal oxide particles The high absolute


(a) (b)

(c)

Figure 2 SEMmicrographs of solvent free SiO2PEGDA composite particles with single vias in square pentagonal and circular cross-sec-

tional shapes Scale bar = 100120583m

values for Al2O3(360mV) and SiO

2(minus209mV) respec-

tively are indications of repulsive interparticle electrostaticinteraction suggesting that particle agglomeration due toelectrostatic attraction is unlikely Consequently one canexpect to achieve uniform stable nanoparticle dispersions asprecursor suspensions and the corresponding high fidelitymetal oxide particles

Two differentmicrofluidic devices with channel depths inthe range of 60ndash80 and 220ndash250120583m are used to synthesizeparticles with the aim of creating particles with differentaxial dimensions Additionally objective lenses (10x and20x) control cross-sectional size Square pentagonal andcircular cross-sectional shapes of particles with single viasare synthesized from the corresponding shape patterned inthe photomask Figures 2(a)ndash2(c) show SEM micrographs ofhigh fidelity monodisperse and solvent free SiO

2PEGDA

composite particles in three cross-sectional shapes Themicrographs in Figure 2 demonstrate that not only arehigh fidelity composite particles synthesized but the cross-sectional shapes and sizes can be manipulated with relativeease For example Figure 2(a) is a square cross section ofparticles measuring 90ndash120120583m in axial and 100ndash110 120583m incross-sectional edge dimensions (220ndash250120583m device 10xobjective lens)The corresponding reduced form (images notshown) which measures 57ndash64120583m and 53ndash59120583m in axial

and cross-sectional edge dimensions is produced using a 60ndash80 120583mdepthmicrofluidic device and a 20x objective lens withthe same size transparent feature in the mask Similar resultsfor pentagonal and circular cross sections have been achievedWith the SFL approach myriad two-dimensional shapes canbe extruded to yield particles Although the particles inFigure 2 have vias in the shape of the overall particles othershapes can be introduced

The synthesis of Al2O3PEGDA composite particles is

also demonstrated Figure 3 shows SEMs of cubic and rod-like composite particles with vias A limitation of theAl2O3PEGDA composite particle synthesis is the UV light

screening due to Al2O3as the weight fraction of Al

2O3

nanoparticles increases However in up to 10wt fractionof Al2O3 it is possible to crosslink the precursor for which

the images are shown in Figure 3 These Al2O3PEG-DA

composite particles measuring sim100ndash110 120583m in axial dimen-sions are synthesized with a lower concentration of PEG-DA(25wt) in order to accommodate the viscosity of the stockAl2O3nanodispersion The composite particle integrity is

illustrated in Figure 4 which shows a uniform wall thicknessfor the particles with square cross sections The thickness ofthe via walls for the square cross sections is estimated to be inthe range of 32ndash35 120583m for the 10x objective and 220-250120583mdevice


Figure 3 SEM micrographs of solvent free Al2O3PEGDA single via composite particles with square and circular cross-sectional shapes

Scale bar = 100120583m

Figure 4 Estimation of the wall thickness for the square crosssection using the Al

2O3PEG-DA particles Scale bar = 100 120583m

The shape of corresponding inorganic particles wasderived from the composites through polymer burn-out andsintering Figures 5(a)-5(b) are a collection of SEMs showingsintered SiO

2particles with vias in square and pentagonal

cross sections The particle fidelity is further demonstratedin the preservation of particle walls even on sintering the viacomposite particles The axial dimensions of the particles forboth cross section shapes were in the range of 75ndash85120583m forthe 10x objective and 220-250120583m device and 35ndash45 120583m forthe 20x objective and 60ndash80120583m device (image not shown)

These particles may have applications for hollowmicronanomaterials based on properties which includehigh specific surface areas and inner voids [22] In manycases such characteristics make the particles superior tothe same sized particle with simple and solid structuresTherefore they are candidates inmany important applicationareas such as catalysis sensors [41] Li-ion batteries [12 15]and drug delivery [42] It is worth noting that the PEG-DAserves two purposes (1) as a binder in the compositeparticles and (2) as a porogenic source in the inorganicparticles Generally sintering reduces particle porosityHowever the presence of the binder (porogen) is expectedto reduce the extent of porosity loss The high magnificationSEM (Figure 6(b)) of the SiO

2particle (inset Figure 6(a))

surface reveals an agglomerated and tightly packed as wellas an uneven particle surface It appears that the compaction

of the composite particles by interparticle electrostaticattractive forces of the constituent inorganic particles isenhanced by the sintering temperature in spite of thedeparting porogen (polymer binder) It is obvious fromthis observation that the final application of the particlesmust be considered when choosing the amount of binderthe sintering temperature the duration of sintering andother synthesis conditions The TEM image of the precursorsuspension (Figure 6(c)) confirms monodisperse constituentSiO2

nanoparticles which are spherically shaped andapproximately 25 nm in diameter Our goal is to produceinorganic particles of unique morphologies indirectly byfirst synthesizing the composites and burning out the binderParticle size reduction is anticipated as the binder leavesand the inorganic component densifies on heat treatmentFor the SiO

2particles the percent change in the particle

volume was calculated and found to range from 58 to 68and 47 to 59 for the 220ndash250120583m and 60ndash80 120583m channelsrespectively Assuming that it is applicable to use the bulkdensity for cristobalite a volume reduction of sim90 fora 20wt loading of SiO

2is expected to correspond to a

sim50 reduction in edge length The difference between theexperimental measurements and theoretical predictionsis attributed to the porosity of the microparticles and thepotential deviation of the nanoparticle density from that ofthe bulk material It is also observed from Figure 6(b) thatthe inorganic component consists of spherically shaped SiO

2

particles of approximately 25 nm diameter which are shownin Figure 6(c)

Figure 7 shows SEM images of Al2O3microparticles in

circular cross sections imaged in progression of magni-fication (Figures 7(a)ndash7(c)) to reveal the microstructuralcharacteristics of the particle surface The micrographssuggest that the microparticles consist of rod-like Al

2O3

nanoparticles aligned in a pattern The structural features ofAl2O3nanoparticles are further revealed in the TEM image

of the precursor suspension in Figure 7(d) Also unlike theSiO2nanoparticles the Al

2O3nanoparticles formed an array

with visible interparticle voids (Figure 7(c)) Consideringthat (1) the weight percent loading of the Al

2O3composite

particles is low and (2) the sintering of Al2O3usually

requires temperatures in excess of 1600∘C one would expect


(a) (b)

Figure 5 SEM micrographs of SiO2single via particles in square and pentagonal cross sections Scale bar = 100 120583m

(a)

(b) (c)

Figure 6 (a) Low mag SEM micrograph of SiO2particles (b) Higher mag SEM of the SiO

2particle surface Scale bar = 200 nm (c) TEM

of the as received Ludox particles Scale bar = 50 nm

highly porous Al2O3microparticles It might also explain

why the theoretical prediction of volume reduction is over95 when only approximately sim60 reduction is measuredexperimentally

Figure 8 shows multivia PEG-DASiO2composite parti-

cles The multiplicity of vias in the particles is a means tocreate particles with ordered porosity as well as providingadditional surface area

The data for BET surface area pore volume and poresize collected by nitrogen physisorption are shown in Table 2Prior to data collection standard microporous and meso-porous reference materials were evaluated to confirm theaccuracy of the instrument The inorganic single via SiO

2

microparticles in square circular and pentagonal cross sec-tions recorded BET surface area values in the range of 56ndash69m2g with pore volume and pore sizes of 008-009 cm3gand 19ndash28 nm respectively Additionally the SiO

2particle

with four vias yielded a BET surface area pore volume andpore size of 114m2g 015 cm3g and 21 nm respectivelySimilarly the Al

2O3via particles recorded specific surface

area values ranging from 73 to 81m2gThe BET data collected for the single and multivia

particles has several implications First the presence of

the vias in these particles makes them superior with respectto surface area for physisorption compared with analogoussolid particles Second based on the data obtained thusfar it is not obvious that the particle cross-sectional shapesinfluenced the BET values Finally for fully densified solidmicroparticles one would expect surface area measurementsless than 01m2g Thus obtaining surface areas from 50 to80m2g prior to optimization suggests that the particleshave potential for applications where structure and highsurface area are important In order to optimize the particlesfor future applications removal of templates and volatilefractions by washing and calcination could increase the par-ticle BET surface area by more than one order of magnitude[43 44] Also control of the binder concentration could beanother method to improve the porosity

4 Conclusions

The SFL method is used to synthesize two-dimensionallyextruded nonspherical PEG-DASiO

2and PEG-DAAl

2O3

composite particles from which the corresponding inorganicparticles are obtained by heat treatmentThe approach repre-sents an indirect application of the SFL technique to obtain


(a) (b)

(c)(d)

Figure 7 SEM micrographs of single via Al2O3microparticles with a circular cross section (a) (b) and (c) are successively higher

magnification images to reveal the microstructural features of (a) (d) TEM of the Al2O3nanoparticles in suspension as received Scale bars

(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm

Figure 8 SEM micrograph of multivia PEG-DASiO2composite

microparticles Scale bars 100 120583m

high fidelity consolidated inorganic particles Microparticlesof SiO

2and Al

2O3with square circular and pentagonal

cross-sectional shapes with vias have been synthesized Twoobjective lens magnifications of 10x and 20x were used todemonstrate the reduction in particle cross-sectional dimen-sions whereas two different channel depths (60ndash80 120583m and220ndash250120583m) controlled the particle axial dimension Highmagnification SEM revealed that the SiO

2microparticles

are formed from compacted spherical SiO2nanoparticles

However the high magnification image for Al2O3revealed

an array of rod-like Al2O3nanoparticles interspersed with

Table 2 BET surface area of single via SiO2 and Al2O3 particles ofdifferent cross sections and multi-via cubic SiO2 particles

(a) SiO2

Particle CrossSection Shape

Specific SurfaceArea(m2g)

Pore Volume(cm3g)

Pore Size(nm)

Square 56 and 114lowast 009 and015lowast

19 and21lowast

Pentagonal 69 008 12Circular 64 008 28Range of particledimension (120583m) 40ndash47

(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)

Square 73 045 35Circular 81 073 29Range of particledimension (120583m) 75ndash90

lowast4 via particle

microvoids BET surface area measurements for the singlevia inorganic particles were 56ndash69m2g for the SiO

2particles


and 73ndash81m2g for the Al2O3particles Additionally multivia

cubic SiO2particles measured 114m2g in BET surface area

as determined by nitrogen physisorption isotherms at liquidnitrogen temperature (77K) The surface areas achievedsuggest that the particles have great potential for applicationsin areas like catalysis and sieving where structure and surfacearea control are important

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

This material is based upon work supported by the NationalScience Foundation under Grant no DMR-0611612 Anyopinions findings and conclusions or recommendationsexpressed in this material are those of the authors and donot necessarily reflect the views of the National ScienceFoundation David Baah gratefully acknowledges a graduatefellowship from Alabama EPSCoR

References

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[2] N S Oltra J Swift A Mahmud K Rajagopal S M Loverdeand D E Discher ldquoFilomicelles in nanomedicine-from flexiblefragmentable and ligand-targetable drug carrier designs tocombination therapy for brain tumorsrdquo Journal of MaterialsChemistry B vol 1 no 39 pp 5177ndash5185 2013

[3] L TaoWHu Y Liu GHuang B D Sumer and J Gao ldquoShape-specific polymeric nanomedicine emerging opportunities andchallengesrdquo Experimental Biology and Medicine vol 236 no 1pp 20ndash29 2011

[4] H Zou S Wu and J Shen ldquoPolymersilica nanocompositespreparation characterization propertles and applicationsrdquoChemical Reviews vol 108 no 9 pp 3893ndash3957 2008

[5] C Burda X Chen R Narayanan and M A El-SayedldquoChemistry and properties of nanocrystals of different shapesrdquoChemical Reviews vol 105 no 4 pp 1025ndash1102 2005

[6] Y-W Jun J-S Choi and J Cheon ldquoShape control of semi-conductor andmetal oxide nanocrystals through nonhydrolyticcolloidal routesrdquo Angewandte Chemie vol 45 no 21 pp 3414ndash3439 2006

[7] A Phuruangrat P Jitrou P Dumrongrojthanath et al ldquoHydro-thermal synthesis and characterization of Bi

2MoO6nanoplates

and their photocatalytic activitiesrdquo Journal of Nanomaterialsvol 2013 Article ID 789705 8 pages 2013

[8] A R Tao S Habas and P Yang ldquoShape control of colloidalmetal nanocrystalsrdquo Small vol 4 no 3 pp 310ndash325 2008

[9] Y Xia Y Xiong B Lim and S E Skrabalak ldquoShape-controlledsynthesis of metal nanocrystals simple chemistry meets com-plex physicsrdquo Angewandte Chemie vol 48 no 1 pp 60ndash1032009

[10] I A Rahman and V Padavettan ldquoSynthesis of Silica nanopar-ticles by Sol-Gel size-dependent properties surface modifi-cation and applications in silica-polymer nanocompositesmdasha

reviewrdquo Journal of Nanomaterials vol 2012 Article ID 13242415 pages 2012

[11] J Hu M Chen X Fang and L Wu ldquoFabrication and applica-tion of inorganic hollow spheresrdquoChemical Society Reviews vol40 no 11 pp 5472ndash5491 2011

[12] X W Lou L A Archer and Z Yang ldquoHollow micro-nano-structures synthesis and applicationsrdquoAdvancedMaterials vol20 no 21 pp 3987ndash4019 2008

[13] A-H Lu E L Salabas and F Schuth ldquoMagnetic nanoparticlessynthesis protection functionalization and applicationrdquoAnge-wandte Chemie International Edition vol 46 no 8 pp 1222ndash1244 2007

[14] J-H Lee ldquoGas sensors using hierarchical and hollow oxidenanostructures overviewrdquo Sensors and Actuators B Chemicalvol 140 no 1 pp 319ndash336 2009

[15] X W Lou Y Wang C Yuan J Y Lee and L A Archer ldquoTem-plate-free synthesis of SnO

2hollow nanostructures with high

lithium storage capacityrdquo Advanced Materials vol 18 no 17 pp2325ndash2329 2006

[16] X W Lou C Yuan and L A Archer ldquoShell-by-shell synthesisof tin oxide hollow colloids with nanoarchitectured walls cavitysize tuning and functionalizationrdquo Small vol 3 no 2 pp 261ndash265 2007

[17] X W D Lou L A Archer and Z Yang ldquoHollow micro-nano-structures synthesis and applicationsrdquoAdvancedMaterials vol20 no 21 pp 3987ndash4019 2008

[18] G Wang T Liu X Xie Z Ren J Bai and H Wang ldquoStruc-ture and electrochemical performance of Fe

3O4graphene

nanocomposite as anode material for lithium-ion batteriesrdquoMaterials Chemistry and Physics vol 128 no 3 pp 336ndash3402011

[19] Y Wang T J Merkel K Chen C A Fromen D E Betts andJ M Desimone ldquoGeneration of a library of particles havingcontrolled sizes and shapes via the mechanical elongation ofmaster templatesrdquo Langmuir vol 27 no 2 pp 524ndash528 2011

[20] Q Zhang B Lin and J Qin ldquoSynthesis of shape-controlled par-ticles based on synergistic effect of geometry confinementdouble emulsion template and polymerization quenchingrdquoMicrofluidics and Nanofluidics vol 12 no 1ndash4 pp 33ndash39 2012

[21] Q Zhang W Wang J Goebl and Y Yin ldquoSelf-templated syn-thesis of hollow nanostructuresrdquo Nano Today vol 4 no 6 pp494ndash507 2009

[22] Y Zhao and L Jiang ldquoHollowmicronanomaterials with multi-level interior structuresrdquoAdvancedMaterials vol 21 no 36 pp3621ndash3638 2009

[23] F Caruso R A Caruso andHMohwald ldquoNanoengineering ofinorganic and hybrid hollow spheres by colloidal templatingrdquoScience vol 282 no 5391 pp 1111ndash1114 1998

[24] V Giridharan Y Yun P Hajdu et al ldquoMicrofluidic platformsfor evaluation of nanobiomaterials a reviewrdquo Journal of Nano-materials vol 2012 Article ID 789841 14 pages 2012

[25] X Wang S Li L Wang et al ldquoMicrofluidic device for control-lable chemical release via field-actuated membrane incorporat-ing nanoparticlesrdquo Journal of Nanomaterials vol 2013 ArticleID 864584 6 pages 2013

[26] C-X Zhao L He S Z Qiao andA P JMiddelberg ldquoNanopar-ticle synthesis in microreactorsrdquo Chemical Engineering Sciencevol 66 no 7 pp 1463ndash1479 2011

[27] D Baah and T Floyd-Smith ldquoMicrofluidics for particle syn-thesis from photocrosslinkable materialsrdquo Microfluidics andNanofluidics vol 17 no 3 pp 431ndash455 2014


[28] S-M Yang S-H Kim J-M Lim and G-R Yi ldquoSynthesis andassembly of structured colloidal particlesrdquo Journal of MaterialsChemistry vol 18 no 19 pp 2177ndash2190 2008

[29] J Lee P W Bisso R L Srinivas J J Kim A J Swiston andP S Doyle ldquoUniversal process-inert encoding architecture forpolymer microparticlesrdquo Nature Materials vol 13 no 5 pp524ndash529 2014

[30] H An H B Eral L Chen M B Chen and P Doyle ldquoSynthesisof colloidal microgels using oxygen-controlled flow lithogra-phyrdquo Soft Matter vol 10 no 38 pp 7595ndash7605 2014

[31] D Baah J Tigner K Bean N Walker B Britton and T Floyd-Smith ldquoMicrofluidic synthesis and post processing of non-spherical polymeric microparticlesrdquoMicrofluidics and Nanoflu-idics vol 12 no 1ndash4 pp 657ndash662 2012

[32] S C Chapin D C Pregibon and P S Doyle ldquoHigh-throughputflow alignment of barcoded hydrogel microparticlesrdquo Lab on aChipmdashMiniaturisation for Chemistry and Biology vol 9 no 21pp 3100ndash3109 2009

[33] D Dendukuri S S Gu D C Pregibon T A Hatton and P SDoyle ldquoStop-flow lithography in a microfluidic devicerdquo Lab ona Chip vol 7 no 7 pp 818ndash828 2007

[34] D Baah T Donnell J Tigner and T Floyd-Smith ldquoStop flowlithography synthesis of non-spherical metal oxide particlesrdquoParticuology vol 14 pp 91ndash97 2014

[35] R F Shepherd J C Conrad T Sabuwala G G Gioia and J ALewis ldquoStructural evolution of cuboidal granular mediardquo SoftMatter vol 8 no 17 pp 4795ndash4801 2012

[36] R F Shepherd P Panda Z Bao et al ldquoStop-flow lithographyof colloidal glass and silicon microcomponentsrdquo AdvancedMaterials vol 20 no 24 pp 4734ndash4739 2008

[37] J-L Blin A Leonard Z-Y Yuan et al ldquoHierarchically meso-porousmacroporous metal oxides templated from polyethy-lene oxide surfactant assembliesrdquo Angewandte Chemie vol 42no 25 pp 2872ndash2875 2003

[38] K Du X Cui and B Tang ldquoTemplate-directed synthesis ofhollow silica beads by an interfacial sol-gel routerdquo ChemicalEngineering Science vol 98 pp 212ndash217 2013

[39] KW Bong S C Chapin D C Pregibon D Baah T M Floyd-Smith and P S Doyle ldquoCompressed-air flow control systemrdquoLab on a ChipmdashMiniaturisation for Chemistry and Biology vol11 no 4 pp 743ndash747 2011

[40] D K Hwang J Oakey M Toner et al ldquoStop-Flow lithographyfor the production of shape-evolving degradable microgelparticlesrdquo Journal of the American Chemical Society vol 131 no12 pp 4499ndash4504 2009

[41] F Iskandar A B D Nandiyanto K M Yun C J Hogan JrK Okuyama and P Biswas ldquoEnhanced photocatalytic perfor-mance of brookite TiO

2macroporous particles prepared by

spray drying with colloidal templatingrdquo Advanced Materialsvol 19 no 10 pp 1408ndash1412 2007

[42] J-F Chen H-M Ding J-X Wang and L Shao ldquoPreparationand characterization of porous hollow silica nanoparticles fordrug delivery applicationrdquo Biomaterials vol 25 no 4 pp 723ndash727 2004

[43] S H Kim B Y H Liu andM R Zachariah ldquoUltrahigh surfacearea nanoporous silica particles via an aero-sol-gel processrdquoLangmuir vol 20 no 7 pp 2523ndash2526 2004

[44] Q Liu P Deshong andM R Zachariah ldquoOne-step synthesis ofdye-incorporated porous silica particlesrdquo Journal of Nanoparti-cle Research vol 14 no 7 article 923 2012

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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NanoparticlesJournal of



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Biomaterials


NanoscienceJournal of

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Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


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MaterialsJournal of


Nano

materials


Journal ofNanomaterials


varying temperatures and pH of precursor solutions are usedas shape size and morphological control parameters follow-ing nucleation and during particle growth [5ndash9] Conditionssuch as slowing down the rate of polycondensation reactionin the sol gel synthesis of SiO

2nanoparticles yield smaller

sized particles whereas larger sized particles are obtained byincreasing the ammonia concentration [10] Other authorshave discussed the use of emulsion droplets hard and softtemplates sacrificial templates and template free approachesfor the synthesis of particles with unconventional structuresand their applications in batteries biomedicine catalysis andsensing [11ndash13] Additionally the independent or combinedapplications of the principles of Oswald ripening and Kirk-endall effects have been used to achieve the synthesis ofgenerally spherical and pseudospherical particles with hollowcavities for applications in gas adsorption and energy storage[14ndash22] Like hollow particles the introduction of small viasor orderedmacropores is a novelmeans of creating structureswith increased surface area [23]

Given the potential of nonspherical particles newapproaches have been developed to provide additional meth-ods to fill the need to synthesize them While microflu-idics technology is diverse in its usage including evalua-tion of nanobiomaterials and controlled release [24 25] itsapplication in particle design and synthesis is unparalleledRecent articles have catalogued specific microfluidic plat-forms particle formation precursors and conditions whichproducemyriad particles with uniquemorphological featuresand production rates [26ndash28] For example the stop flowlithography (SFL) technique used in this study producesparticles at a rate of 02 ghr and depends on the free radicalpolymerization of the precursor onto which UV light isprojected using a microscope SFL is limited to extrusionof two-dimensional particles and therefore is an excellentmethod for synthesis of nonspherical particles for the currentand potential applications that benefit from nonsphericalparticle shapes SFL controls the particle axial dimensionand the cross-sectional area using the microchannel depthand objective lens magnification respectively The versatilityof this method extends to the ability to synthesize encodedparticles for microRNA detection [29] with dimensions assmall as 800 nm realized [30] In previous work by the authorand others the SFL technique has been used to demonstratethe synthesis of nonspherical organic particles and theircomposites [31ndash33] as well as metal and metal oxide particles[34ndash36] However niche application requirements of metaloxide particles demand particles with ordered porosity whichpossess important physical properties such as high surfacearea favorable to heterogeneous catalysis supports sievesand adsorbents [37 38]

In this paper we report the synthesis of two-dimen-sionally extrudedmicroparticles with single andmultiple viasand different cross-sectional shapes using the SFL methodThe particles reported in this paper consist of high fidelityand solvent free PEG-DASiO

2and PEG-DAAl

2O3com-

posite particles and their corresponding inorganic particlesobtained through binder burn-out In this work microstruc-tured particles are those in which vias are introducedwith theaim of imparting additional surface area The corresponding

particle nanostructure results from the inorganic nanopar-ticles used to synthesize the microparticles This paper issignificant because it extends our work on the synthesis ofsolid metal oxide particles using SFL to microstructuredparticles with potential applications in the areas of catalysissupports sieves and adsorbents Also to our knowledge itprovides the first detailed characterization of the nanostruc-ture of these types of particles SiO

2and Al

2O3are common

catalyst support materials and were chosen because SiO2has

a low melting point and Al2O3was simultaneously being

investigated for abrasives applications However in theorythis method is applicable to anymetal oxide or other materialthat can be acquired in nanoparticle form and has a meltingpoint higher than 600∘C the temperature above which allpolymer binder is removed

2 Experimental

21 Materials Precursor suspensions are prepared using acommercial SiO

2dispersion in water (Ludox AS-40 or Ludox

CL-X colloidal silica) poly(ethylene glycol) diacrylate 2-hydroxy-2-methylpropiophenone hydrogen peroxide andascorbic acid all supplied by Sigma Aldrich as well as analuminum oxide nanodispersion (Al

2O3 20 wt 30 nm)

supplied by US Research Nanomaterials Inc The parti-cle synthesis was conducted using geometrically patternedphotomasks and in microfluidic devices whose fabricationmethods were described in a preceding work [31 39]

22 Particle Synthesis and Collection Figure 1 is a schematicillustration of the particle synthesis workstation Patternedphotomasks mask the UV light from a source This pho-tomask is inserted in the field stop of an upright micro-scope (Olympus BX51WF Microscope) and changed whennecessary to change the imposed cross-sectional shapes ofthe particles The UV light is focused onto the precursorflow stream using a 10x or 20x microscope objective lens Inaddition to light focusing the objective controls the particlecross-sectional size with the particle size decreasing as theobjective magnification increasesThe axial dimension of theparticles is controlled by the microfluidic channel depth In atypical operation a three-way solenoid valve regulates the air-pressure-driven precursor solution supply to themicrofluidicdevice mounted on the stage of a microscope The precursoris prepared bymixing aliquots of the commercial suspensionsof the inorganic nanoparticles with PEG-DA to constitutesuspensions containing 10wt Al

2O3and 20wt SiO

2in

30wt PEG-DA The goal was to use the highest possibleconcentration of nanoparticles in the suspension but theconcentrationwas limited by excessive fluid resistance to flow(high viscosity) caused by the particle loading as well as lightscattering and consequent inadequate binder crosslinking inspite of index matching The source of precursor supply isa 10mL syringe with the tip connected to the microfluidicdevice through ethyl vinyl acetate microbore tubing of 00410158401015840ID (WU-06492-04 Cole Parmer)The light source is a Lumen200 (Prior Scientific) equipped with a 200 watt metal arcbulb connected to a Shutter Driver (VCM-D1 Uniblitz)whose opening and closure are synchronized with those of


UV source

Patternedmask

Microfluidicdevice

Shutter


Particle outlet



Shutter driver





2particles


2O3composite




SiO2 Al2O3








(a) (b)

(c)







2PEGDA






2O3






















2




2O3





2O3composite




(a) (b)


(a)

(b) (c)









2


2particle






4 Conclusions


2and PEG-DAAl

2O3



(a) (b)

(c)(d)



(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm




2and Al



2microparticles





(a) SiO2



Pore Volume(cm3g)

Pore Size(nm)


19 and21lowast


(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)




2particles







Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




UV source

Patternedmask

Microfluidicdevice

Shutter


Particle outlet



Shutter driver





2particles


2O3composite




SiO2 Al2O3








(a) (b)

(c)







2PEGDA






2O3






















2




2O3





2O3composite




(a) (b)


(a)

(b) (c)









2


2particle






4 Conclusions


2and PEG-DAAl

2O3



(a) (b)

(c)(d)



(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm




2and Al



2microparticles





(a) SiO2



Pore Volume(cm3g)

Pore Size(nm)


19 and21lowast


(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)




2particles







Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c)







2PEGDA






2O3






















2




2O3





2O3composite




(a) (b)


(a)

(b) (c)









2


2particle






4 Conclusions


2and PEG-DAAl

2O3



(a) (b)

(c)(d)



(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm




2and Al



2microparticles





(a) SiO2



Pore Volume(cm3g)

Pore Size(nm)


19 and21lowast


(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)




2particles







Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




















2




2O3





2O3composite




(a) (b)


(a)

(b) (c)









2


2particle






4 Conclusions


2and PEG-DAAl

2O3



(a) (b)

(c)(d)



(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm




2and Al



2microparticles





(a) SiO2



Pore Volume(cm3g)

Pore Size(nm)


19 and21lowast


(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)




2particles







Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)


(a)

(b) (c)









2


2particle






4 Conclusions


2and PEG-DAAl

2O3



(a) (b)

(c)(d)



(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm




2and Al



2microparticles





(a) SiO2



Pore Volume(cm3g)

Pore Size(nm)


19 and21lowast


(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)




2particles







Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c)(d)



(a) = 50 120583m (b) = 10 120583m (c) = 100 nm and (d) = 100 nm




2and Al



2microparticles





(a) SiO2



Pore Volume(cm3g)

Pore Size(nm)


19 and21lowast


(b) Al2O3



Pore Volume(cm3g)

Pore Size(nm)




2particles







Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









Acknowledgments


References








2MoO6nanoplates
















3O4graphene






































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






























CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Research Article Stop Flow Lithography Synthesis and...

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