Electrostatic Layer-by-Layer Nano-Assembly: Films ...hj56/PDFfiles/2005/SPIE paper soft...attraction (Figure 1). Simplicity and universality of LBL assembly makes it an attractive

Electrostatic Layer-by-Layer Nano-Assembly: Films, Cantilevers, Micropatterns and Nanocapsules

Dinesh Kommireddy, Jingshi Shi, Xiaodong Yan, Haifeng Ji, Yuri Lvov

Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272

ABSTRACT Layer-by-layer (LbL) nanoassembly in combination with traditional lithography and microfluidics was applied for the fabrication of ultrathin microcantilevers and for the modification of microchannel surface. Hundreds of cantilevers were fabricated on a silicon wafer simultaneously. The purpose is to develop chemical/biosensor arrays for parallel, massive data gathering. Microcantilever optical deflections were measured using a four-quadrant AFM head with integrated laser and position sensitive detector. In the second part, laminar flow fabrication of interpolyelectrolyte complexes was studied inside a microchannel. Polyelectrolyte micropatterns were studied using fluorescent confocal microscopy. Filament like, 15 µm, interpolyelectrolyte microstripes were formed at flow rate higher than 0.01 mL·min-1 and concentrations of the initial polyelectrolytes below 1 mg·mL-1. New, soft micropatterning technique for the anisotropic modification of polyelectrolyte nanocapsules was also demonstrated. The microchannel surface was made sensitive to pH by coating the surface of the channel with a pH sensitive dye using LbL assembly to control the reaction.

Keywords: Layer-by-Layer assembly, Cantilevers, Microfluidics, Lithography

1. INTRODUCTIONLayer-by-Layer Nanoassembly Technologies bridging the gap between the bottom-up nanoassembly and top-down micromanufacturing are of special interest due to the high density of structures that can be produced with precise control over architecture at the nanoscale. Layer-by-Layer (LbL) self-assembly is a prospective nanofabrication technique to produce multilayer structures with precision of one nanometer through alternate adsorption of oppositely charged components (polymers, enzymes and nanoparticles) primarily via electrostatic attraction (Figure 1). Simplicity and universality of LBL assembly makes it an attractive alternative for the production of thin film nanostructured materials. Originally tested for oppositely charged micron sized colloids [1], almost thirty years later it was successfully extended to polyelectrolyte pairs [2] and many other nanoblocks [3-5]. Availability of aqueous dispersions of charged nanoparticles extended the application of LBL assemblies to the preparation of hybrid organic/inorganic multilayer composites. Sequential absorption of inorganic colloids and oppositely charged polyelectrolytes has been demonstrated for semiconductor nanoparticles[6], metal nanoparticles [7-8], silica particles[9-10], and exfoliated layered compounds[5]. Surface modification has been achieved for corrosion control [11] and biocompatibility of various surfaces [12-13] using the LbL method.

Invited Paper

Nanofabrication: Technologies, Devices, and Applications, edited by Warren Y-C. Lai,Stanley Pau, O. Daniel López, Proc. of SPIE Vol. 5592 (SPIE, Bellingham, WA, 2005)

0277-786X/05/$15 · doi: 10.1117/12.568046

120

-

-

-

-

+

+

+

+

Polyanion

Polycation

Nanoparticles

Enzymes

---

++-

-

-

-

+

+

+

+

+

+

+

+

-

-

-

--

-

-

-

Figure 1. Scheme of Layer-by-Layer assembly on a planar substrate.

Nanocomposite MicrocantileversRecent advances in designing and fabricating microcantilever beams capable of

detecting extremely small forces, mechanical stresses, and mass additions offer thepromising prospects of chemical, physical, and biological sensing with unprecedented sensitivity and dynamic range[14-15]. The cantilevers are microscopic bars free to moveat one end and fixed at the opposite end which are commonly produced using standard silicon microfabrication techniques. A cantilever array enables multiple experiments to be performed simultaneously and expedites the experimental process. We produced microcantilevers with LbL assembly providing them with additional nanoscale features.

Microfluidic Device Integration of soft-lithography and microfludics has offered new fluidic devices

used in various fields of chemistry and biology[16]. Laminar flow of multiple fluidsinside one channel allows them to move individually, the only interaction between themis slow molecular diffusion across the interface[17-18]. Usually, laminar flow synthesis is carried out inside microchannels prepared by softlithographic techniques. Multiphaselaminar flow has been used for microfabrication of lipid bilayers inside capillaries[19],patterned delivery of cells and proteins[20], selective deposition of material insidemicrofludic channels[17-21], separation[22], detection[23], and as membraneless fuel cell[24-25]. We used microchannel device coated with nanofilms for chemical reaction and patterning of microspheres.

This paper summarizes the results of combining LbL assembly withmicrofabrication and microfluidics. LbL was used for the construction of the microcantilevers for chemical/bio sensing. It is expected that these microarray assays willbe able to provide massive, parallel platforms for data gathering. This design introduces new features to nanocomposite microcantilever because they are much thinner and more flexible than existing solid silicon or metal microcantilevers and can have unique nanocomposite organizations. Thickness, flexibility and functionality of LbL multilayerscan be adjusted by changing the number of monolayers of clay, polymers, nanoparticles, and other nanoblocks (such as DNA and enzymes). Magnetic field can be detected by integrating magnetic nanoparticles into the composite microstrips. In the second part, the micron-resolved synthesis, of interpolyelectrolyte complexes inside a laminar flow

Proc. of SPIE Vol. 5592 121

microreactor was demonstrated on examples of poly(dimethyldiallylammonium chloride) and poly(styrene sulfonate). The width of the polyelectrolyte complex and its position in the channel microreactor can be adjusted by the concentration and flow rate of individual components. The surface of the channels was made sensitive to pH by layering the surface of the channel with a pH sensitive dye, 1-hydroxypyrene-3, 6, 8- trisulfonic acid (HPTS).

2. MATERIALS AND METHODS

Microcantilevers Poly(dimethyldiallyl ammonium chloride), MW 200-300K (PDDA, Aldrich), and sodium poly (styrenesulfonate), MW 70,000 (PSS, Aldrich), were used at a concentration of 1.5 - 3 mg/mL. Montmorillonite clay (Sigma) and magnetite powder were dispersed in DI water at a concentration of 5 mg/mL. These suspensions were stirred up and ultrasonicated for 30 min before use. Magnetite nanoparticles (Fe3O4/Fe2O3,14 nm in diameter) were obtained from PolySciences Corporation.

Positive photoresist (PR1813, Shipley) was coated on a silicon wafer with 2 moxide. The photoresist was subsequently exposed to UV light through a mask. After developing and removing the exposed oxide, inductively coupled plasma (Alcatel) etching was done on this patterned wafer to form trenches to easily divide the wafer. Photoresist was again applied on the wafer and a second mask was utilized to transfer channels onto the photoresist. The photo resist was irradiated again through the third mask to project the image of the cantilever beams on the photoresist. LbL assembly of clay and magnetite nanoparticles was performed on this patterned substrate. Two precursor bilayers of PDDA and PSS were deposited to provide a uniformly charged surface. The subsequent coating of clay was optimized to make the multilayer strong and flexible. The sequence of assembly being [PDDA (10 min) +PSS (10 min)]2 cycles +[PDDA (10 min) + Fe3O4-Fe2O3 (15 min) +PDDA (10min) +Clay(8 min)]8 cycles. The intermediate rinsing and drying after each immersion was necessary. Subsequently, 20nm titanium and 20nm gold were deposited on the multilayer. The wafer with the stratified layers of polyelectrolyte, magnetic nanoparticles, clay powder, titanium and gold was soaked in the developer solution to remove the photoresist and the multilayers above it. The wafer was then placed in acetone for 24 hours to obtain free-standing cantilevers. The wafer was then divided to obtain cantilever bodies (Fig.2). Example of a thin film assembled to build up multilayers with alternating montmorillonite (clay) nanoparticles and cationic PEI or PDDA is shown in Fig. 3.

122 Proc. of SPIE Vol. 5592

L i t h o g r a p h y LbL

1500um

3500um

1500um

3500um

Figure 2. Fabrication procedure of microcantilevers by lithography and LbL assembly(From the beginning: Wafer substrate after ICP etching; channel formation; cantileverimage; LbL deposition; Lift-off; cantilever release; a divided microcantilever with base).

ba

Figure 3. Example of a free standing montmorillonite/PDDA multilayer film used for cantilever preparation, a. SEM image of a piece of montmorillonite sheet; b. Cross-

section of the montmorillonite sheet.

Microfluidics Poly(dimethyldiallylammonium chloride) (PDDA, MW ~ 200,000, Aldrich), poly(allylamine hydrochloride) (PAH, MW 10,000, Aldrich), which are positivepolyelectrolytes at pH<11.5 and pH<8.5, respectively, and poly(styrene sulfonate) (PSS,MW 70,000, Aldrich), which is a negative polyelectrolyte at pH> 1.5, were used for the formation of polyelectrolyte complexes. HPTS (SIGMA-H1529) solution and PDDA solution were premixed in the ratio of 1:4 respectively according to the proceduredescribed in Ref.26. HPTS-PDDA was layered with PSS inside the channel by LbLassembly. For confocal fluorescence microscopy investigations PDDA and PAH were labeled with fluorescein isothiocyanate (FITC) and PSS was labeled with tetramethylrhodamine isothiocyanate (TRITC) according to the procedure described in Ref. 27.

The fabrication of microchannels is a straight forward lithographic process. Briefly lithography on a silicon wafer was done using a thick layer of positive photoresist.The wafer was then etched using inductively coupled plasma etching (Alcatel, 601E, USA) to get channels of about 200 µm in depth. Resulting wafer pattern was used as a mold for the subsequent hot-embossing (HEX01/LT, JENOPTIK Mikrotechnik GmbH,


Jena, Germany) of poly(methyl methacrylate) (PMMA) sheet. This hot-embossing step transfers the channels onto the PMMA sheet as raised lines of desired width. PMMA replica was used as the final mold to form microchannels in polydimethyl siloxane (PDMS) membrane. To obtain final PDMS microchannels, PDMS (Sylgard 184 siliconeelastomer) and the curing agent (Sylgard 184 curing agent) were mixed in 10:1 ratio, poured onto the PMMA replica and heated at 800C for 30 minutes.

3. CHARACTERIZATION

Inductively coupled plasma etch system (Alcatel) was used for dicing the wafer, DC-RF magnetron sputter deposition system (Uni-Film Technology) was used for deposition of titanium and gold on the surface of the microstrip. For analysis of the microcantilever structures, we used a scanning electron microscope (SEM, AMRAY), and Optical microscope (Olympus Japan). Microphotographs of complex polyelectrolyte microstructures (in bright field mode) were made using NIKON Eclipse TS100 (Nikon,Japan) optical microscope. Confocal fluorescence microscopy images were obtained on a Leica TCS SP confocal fluorescent microscope (Leica, Germany) equipped with 10x and 63x oil immersion objectives. Theoretical modeling and analysis for “Y”-shaped channels was done by CoventorWare software, a fully integrated MicroelectromechanicalSystems (MEMS) design environment.

4. FORMATION OF MICROCANTILEVERS AND ARRAYS FORCHEMICAL/BIO SENSING

The size of the microcantilevers is 200µm×100µm (Fig.4). The thickness of the microstrip measured using tencor profiler was 240nm. The cantilevers are highly flexible, and can bend over 90 degree by moving a permanent magnet of 400 Oe, 0.5cm abovethem and restore immediately once the magnetic field was removed[28].

canning electron microscopy image of a microcantilever (left) and optical microscope image of microcantilever array (right)

500 um

Figure 4. S


Chemical sensing using microcantileversThe apparatus used for deflection measurements is shown in Fig. 5. The

xperiments were carried out in a flow-through glass fluid cell that holds themreplacement of the measured utilizing n AFM head by monitoring the position of a laser beam reflected off the surface of the antilever onto a four-quadrant photodiode (position-sensitive detector)[29]. The inset in igure 4 shows the real picture when the cantilever was incorporated into the system.nce the molecules (in preliminary experiments – glucose oxidase) are absorbed through

ucose oxidase will react with glucose in the

eicrocantilever. The volume of the glass cell is 0.3 cm3 which ensures the fast

solution. The deflection of the microcantilever wasacfOLbL assembly onto the cantilever surface, glsolution. The cantilever will bend due to the stress induced by the reaction and thedeflection can be detected.

Figure 5. Schematic representation of the instrument showing the method of measuringcantilever deflection and the scheme for introducing solutions onto the cantilever.

5. INTERPOLYELECTROLYTE COMPLEX FABRICATION

Experimental setup is schematically represented in Figure 6. Microfluidic reactorsystem is composed of two inlet channels (1, 2), reactor microchannel (3), and one outlethannel (4). Before the experiment, the microfluidic system was filled with water. Then,

ewith arstream in reactor microchannel. S and Y types of the reactor microchannels were mployed to demonstrate the fabrication of polyelectrolyte microstripes of different shape

cach reagent solution was gently pumped into its inlet by a KdScientific pump equipped

two syringes (5). Solutions flowing in inlet channels converge into a single lamin

eby microfluidic laminar approach.


Figure 6. Schem ental setup: 1, 2 - inlet channels; 3 - reactor microchannel; 4 - outlet channel; 5 - pump system equipped

,there is no turbulence at the interface between them; thus, blurring of the interface

lyte micropattern inside reactor microchannel. The Reynolds number of

atic illustration of the laminar flow pattering experim

with two syringes. When two or more liquids flow in parallel at low values of Reynolds number

between fluid streams occurs primarily due to diffusion of particles in transversedirection. Laminar flow controls the spatial delivery of two oppositely chargedpolyelectrolytes within the microchannel and allows the reaction to be exclusivelyconducted at the liquid/liquid interface. The width of resulting interpolyelectrolytecomplex is determined by the diffusion of the reagents across the interface and flow rates of inlet streams. Reagents with high diffusion coefficient (ions, small molecules) form broad micropattern inside microchannel while the reaction between high molecularweight polyelectrolytes, proteins, DNA, etc. results in thin (several microns)micropatterns.

PDDA and PSS inlet solutions of different concentration were used to obtain interpolyelectroinlet laminar streams was <1 that is far below the critical Reynolds number (2100) at which the turbulence starts. 1 mg·mL-1 PDDA and 1 mg·mL-1 PSS solutions, flowing at the same flow rate (0.01 mL·min-1), react at the liquid/liquid interface resulting in PDDA/PSS interpolyelectrolyte complex positioned precisely in the middle of the reactor microchannel (Fig. 7a). Similar phenomenon can be observed taking more geometricallycomplicated S-type microchannel instead of simple Y-type example. Interpolyelectrolytecomplex entirely mimics the shape of the microchannel (Fig. 7b). The only limitation ofthe channel geometry is to keep the flow of two streams parallel and laminar. The widthof interpolyelectrolyte complex is mostly affected by PDDA and PSS concentrations in inlet solutions and can be varied from 10-20 m (for 2 mg·mL-1 solutions or more) to 6-8

m (for 0.1 mg·mL-1 solution). The ratio between concentrations of the reagents in inlet channels does not influence the width of the resulting microstripe while the total flow rate(within the range 0.01 - 2 mL·min-1) has a minor control over it. Numerical simulation of interpolyelectrolyte interaction inside “Y”-type reactor microchannel was carried outusing CoventorWare MEMS design environment to determine the effect of different flow rates of both PDDA and PSS laminar streams on the position of PDDA/PSS complex.The results of the simulation with the same flow rate in both inlet channels showed theformation of the interpolyelectrolyte complex in the center of the reactor microchannel(Fig. 7d) while different flow rates shift the complex towards the side of the lower flow


rate stream (Fig. 7f). This can be attributed to the volume of the fluid flowing in thechannel. As the flow rate of one stream increases, the volume of the channel occupied by

Figure 7. (a-c) - optical micrographs of PDDA/PSS microstructure obtained in the laminar flow channel microreactor. (d, f) - numerical simulation of the PDDA - PSS

this fluid also increases resulting in pushing the diffusion zone towards the lower flowrate side. PDDA/PSS complex formation at different inlet flow rates (0.05 mL·min-1

PDDA, 0.01 mL·min-1 PSS) confirms the simulation results (Fig. 7c). Changing the flow rate in one channel by five times moves the liquid/liquid interface in the direction to thelow flow rate stream. Thus, the flow rate ratio between inlet streams is an easy and powerful parameter to adjust the position of interpolyelectrolyte micropattern in reactormicrochannel. Confocal fluorescence microscopy visualized the resulting PDDA/PSS pattern on the glass slide (Fig. 7e), where 20 m PDDA/PSS strips are separated by20 m gaps.

interaction inside the channel microreactor. (e) - Confocal fluorescent image of a PDDA/PSS micropattern on a glass support. PDDA was labeled with FITC.

The Micropattern was formed by changing the flow rate ratio from1:1(central microstrip) to 1:1.8 and 1.8:1(lower and upper microstrips).


pH sensitive channels e made sensitive to pH by layering pH sensitive material

ade using LbL

nisotropic N in pattern microfabrication offers the possibility to

The microchannels wer(HPTS). PDDA-HPTS and PSS solutions were made in sodium acetate buffer at pH 5. Seven bi-layers of PDDA-HPTS and PSS were layered inside the channel. A final layer of PDDA-HPTS was layered. This was achieved by electrostatic layer-by-layer assembly. HPTS is a fluorescent pH indicator. By layering the inside of the channel with such material, biological/chemical reactions can be carried out inside the channel and the pH atdifferent points can be estimated by monitoring the intensity change of the pH sensitive material (HPTS). In Fig 8, a channel prepared by LbL assembly is shown, and itdemonstrates higher fluorescent intensity at pH 10 and lower intensity at pH 4. Thesolution with pH 10 was pumped from the lower inlet channel and the solution with pH 4was pumped from the upper channel, the inlet channel in the middle was sealed.

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800

Distance ( m)Fl

uors

cenc

e In

tens

ity (A

u)

Figure 8. Confocal fluorescent image of the pH sensitive channel massembly and the intensity profile inside the channel (inset).

anocapsulesAMicron scale resolution

employ microfluidic approach for anisotropic modification of polyelectrolyte nanocapsules, for imparting their advanced release properties and protection ability.Initial Rhodamine-labelled melamine formaldehyde template microparticles wereadsorbed inside reactor microchannel. Then, in one inlet channel FITC-labeled dextran(Mw~10000, 1 mg·mL-1 solution) was introduced while distilled water was introduced from the other. Selective deposition of labeled dextran macromolecules on the part of the template microparticles adsorbed in the center of the channel microreactor was found (Fig. 9).


6. Conclusions

Nanocomposite microcantilever arrays were fabricated using traditional

size differentlyele

. ACKNOWLEDGEMENTS

This work is partially supported by NSF #0210298, and NIH-1RO1 EB00739-01

Figure 9. a) Anisotropic micropatterning of rhodamine labeled microparticulate templateswith fluorescein-labeled dextran macromolecules. b) confocal fluorescent image.

c) overlaid fluorescent image of the resulting anisotropic microparticles.

lithography and layer-by-layer assembly. These microcantilevers are highly flexible dueto the special properties of polymers and nano-ceramics used, providing a new route for the construction of sensitive chemical sensors. Work to modify the surface ofmicrocantilever by LbL self-assembly for chemical sensing is in progress.

Laminar flow microfabrication approach was employed to synthepo ctrolyte complexes. Filament like interpolyelectrolyte micropatterns were formedfrom poly(dimethyldiallyl ammonium chloride) and poly(styrene sulfonate). Multiphaselaminar flow microfabrication allows to produce 5-10 µm micropatterns containing sensitive biological objects on the surface of any appropriate substrate (i.e. glass,polyimide, silicon). pH sensitive channels made with LbL coating allows, in thismicrofluidic device, to monitor conditions in a particular portion of the channel for chemical and biological reactions. The pH at a certain point in the channel is connectedwith the reaction rate. Demonstrated microfabrication procedure is especially useful in handling delicate and sensitive fabrication of bioactive microarrays.

7

grants. Any opinions, findings, and conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the view of the NationalScience Foundation.


REFERENCES

. R. J. Iler, “Multilayers of colloidal particles”, J. Colloid Interface Sci., 1966, 21, 569.

3. osites”,

4. er, “Combination of polycation/polyanion self-assembly

5. N. A. Kotov, I. Dekany, J. H. Fendler, “Layer-by-Layer Self-Assembly of hem., 1995,

6. T. Cui, F. Hua and Y. Lvov, “FET fabricated by layer-by-layer nanoassembly”, IEEE

7. w, R.E. Geer, R. Shashidhar, J. M.

8. noassembly and

9. bly of 1997,

10. J. Shi, F. Hua, T. Cui, and Y. Lvov, “Temperature Effect on Layer-by-Layer 6.

11. e

12. elective Deposition

13. M. C. Berg, S. Y. Yang, P. T. Hammond, and M. F. Rubner, “Controlling mammalian

14. T. Thundat, R. J. Warmack, G. Y. Chen, D. P. Allison, “Thermal and Ambient-tt.,

15. T. Thundat, E. A. Wachter, S. L. Sharp, R. J. Warmack, “Detection of Mercury

16. icated

17., 83.

18. ite,

12. Y. Lvov, G. Decher, H. Haas, H. Mohwald, A. Kalachev, “X-ray analysis of ultrathin

polymer films self-assembled onto substrates”, Physica B, 1994, 198, 89. G. Decher, “Fuzzy nanoassemblies: Toward layered polymeric multicompScience, 1997, 277, 1232. Y. Lvov, F. Essler, G. Dechand Langmuir-Blodgett transfer for the construction of superlattice films”, J. Phys. Chem., 1993, 97, 13773.

Polyelectrolyte-Semiconductor Nanoparticle Composite Films”, J. Phys. C99, 13065.

Transactions on Electron Devices, 2004, 51, 506. J. Schmitt, G. Decher, W. J. Dressick, S. L. BrandoCalvert, “Metal Nanoparticle/Polymer Superlattice Films: Fabrication And Control Of Layer Structure”, Abstr. Pap. Am. Chem. Soc., 1996, 212, 210. J. Shi, T. Cui , “Fabrication of indium resistors by layer-by-layer namicrolithography techniques”, Solid State Electronics, 2003, 47, 2085.

Y. Lvov, K. Ariga, M. Onda, I. Ichinose, T. Kunitake, “Alternate AssemOrdered Multilayers of SiO2 and Other Nanoparticles and Polyions”, Langmuir,13, 6195.

Self-Assembly of Linear Polyions and Silica”, Chemistry Letters, 2003, 32, 31 Tarek R. Farhat and Joseph B. Schlenoff, “Corrosion Control Using PolyelectrolytMultilayers”, Electrochem. and Solid-State Let., 2002, 5, B13. H. Ai, Y. Lvov, D. Mills, J. Alexander, S. Jones, “Coating and Sof Nanofilm on Silicone Rubber for Cell Adhesion and Growth”, Cell Biochemistry and Biophysics, 2003, 38, 103.

Cell Interactions on Patterned Polyelectrolyte Multilayer Surfaces”, Langmuir, 2004,20, 1362.

Induced Deflections of Scanning Force Microscope Cantilevers”, Appl. Phys. Le1994, 64, 2894.

Vapor Using Resonating Cantilevers”, Appl. Phys. Lett., 1995, 66, 1695. A. Y. Fu, C. Spence, A. Scherer, F. H. Arnold, S. R. Quake, “A microfabrfluorescence-activated cell sorter”, Nature Biotechnology, 1999, 17, 1109. P. J. A. Kenis, R. F. Ismagilov, G. M. Whitesides, “Microfabrication Inside Capillaries Using Multiphase Laminar Flow Patterning”, Science, 1999, 285 P. J. A. Kenis, R. F. Ismagilov, S. Takayama, G. M. Whitesides, S. Li, H. S. Wh“Fabrication inside Microchannels Using Fluid Flow”, Acc. Chem. Res., 2000, 33, 841.


19. L. Kam, S. G. Boxer, “Spatially Selective Manipulation of Supported Lipid Bilayers by Laminar Flow: Steps Toward Biomembrane Microfluidics”, Langmuir, 2003, 19, 1624.

20. R. S. Kane, S. Takayama, E. Ostuni, D. E. Ingber, G. M. Whitesides, “Patterning Proteins and Cells Using Soft Lithography”, Biomaterials, 1999, 20, 2363.

21. S. L. R. Barker, D. Ross, M. J. Tarlov, M. Gaitan, L. E. Lacascio, “Control of flow direction in microfluidic devices with polyelectrolyte multilayers”, Anal. Chem., 2000,72, 5925.

22. J. Oakey, J. Allely, D. W. M. Marr, “Laminar-Flow-Based Separations at the Microscale”, Biotechnol. Prog., 2002, 18, 1439.

23. A. Hatch, A. Kamholz, K. Hawkins, M. Munson, E. Shilling, B. H. Weigl, P. Yager, “A rapid diffusion immunoassay in a T-sensor”, Nature Biotechnology, 2001, 19, 461.

24. R. Ferrigno, A. D. Stroock, T. D. Clark, M. Mayer, G. M. Whitesides, “Membraneless Vanadium Redox Fuel Cell Using Laminar Flow”, J. Am. Chem. Soc., 2002, 124, 12930.

25. S. Takayama, J. C. Mcdonald, E. Ostuni, M. N. Liang, P. J. A. Kenis, R. F. Ismagilov, G. M. Whitesides, “Patterning cells and their environments using multiple laminar fluid flows in capillary networks”, Proc. Natl. Acad. Sci. USA, 1999, 96, 5545.

26. D. Terala, “Micro/nano-fabrication of cell culture scaffolds integrated with fluorescent sensors”, Masters Thesis, Louisiana Tech University, 2004.

27. G. Ibarz, L. Dahne, E. Donath, H. Mohwald, “Smart Micro- and Nanocontainers for Storage, Transport, and Release”, Adv. Mater., 2001, 13, 1324.

28. F. Hua, T. Cui, Y. M. Lvov, “Ultrathin Cantilevers Based on Polymer-Ceramic Nanocomposite Assembled through Layer-by-Layer Adsorption”, Nano Letters, 2004,4, 823.

29. G. Y. Chen, T. Thundat, E. A. Wachter, R. J. Warmack, “Adsorption-induced surface stress and its effects on resonance frequency of microcantilevers”, J. Appl. Phys.,1995, 77, 3618.


Documents

Electrostatic Layer-by-Layer Nano-Assembly: Films ...hj56/PDFfiles/2005/SPIE paper soft...attraction (Figure 1). Simplicity and universality of LBL assembly makes it an attractive