R EPORTS of the autocorrelation scans also consistently shows an amplitude enhancement by more Nanowire Nanosensors for than 20% when the phase lock is activated. An important issue will be to demonstrate Highly Sensitive and Selective control over the phase profile across the en- tire synthesized spectrum, namely pulse shaping. For example, a flat spectral phase Detection of Biological and profile is a prerequisite for generating an ultrashort pulse while arbitrary shape is need- Chemical Species ed for coherent control applications. Using Yi Cui,1* Qingqiao Wei,1* Hongkun Park,1 Charles M. Lieber1,2† the current control scheme, the carrier-enve- lope slip phases of the two lasers track each Boron-doped silicon nanowires (SiNWs) were used to create highly sensitive, ⌬␾ ϭ⌬␾ other ( 1 2). There remains, of course, real-time electrically based sensors for biological and chemical species. Amine- a static phase difference between the two and oxide-functionalized SiNWs exhibit pH-dependent conductance that was ␾ Ϫ␾ lasers, namely ( 1 2). However, this stat- linear over a large dynamic range and could be understood in terms of the ic phase can be controlled through an appro- change in surface charge during protonation and deprotonation. Biotin-mod- priate phase offset introduced in the carrier ified SiNWs were used to detect streptavidin down to at least a picomolar heterodyne beat detection, for example, concentration range. In addition, antigen-functionalized SiNWs show reversible through phase shift of the radio frequency antibody binding and concentration-dependent detection in real time. Lastly, signal driving the AOM. This phase-compen- detection of the reversible binding of the metabolic indicator Ca2ϩ was dem- sated spectrum can then be used in a pulse- onstrated. The small size and capability of these semiconductor nanowires for shaping device to generate the desired pulse sensitive, label-free, real-time detection of a wide range of chemical and bi- waveform. ological species could be exploited in array-based screening and in vivo diagnostics. References and Notes 1. M. H. Anderson, J. R. Ensher, M. R. Matthews, C. E. Planar semiconductors can serve as the basis suggested that direct binding of electron- Wieman, E. A. Cornell, Science 269, 198 (1995). 2. C. A. Sackett et al., Nature 404, 256 (2000). for chemical and biological sensors in which withdrawing NO2 or electron-donating NH3 3. R. S. Judson, H. Rabitz, Phys. Rev. Lett. 68, 1500 detection can be monitored electrically and/or gas molecules to the NT surface chemically (1992). optically (1–4). For example, a planar field gated these devices. However, several prop- 4. D. E. Spence, P. N. Kean, W. Sibbett, Opt. Lett. 16,42 (1991). effect transistor (FET) can be configured as a erties of NTs could also limit their develop- 5. J. P. Zhou et al., Opt. Lett. 19, 1149 (1994). sensor by modifying the gate oxide (without ment as nanosensors, including the follow- 6. R. Ell et al., Opt. Lett. 26, 373 (2001). gate electrode) with molecular receptors or a ing: (i) existing synthetic methods produce 7. R. R. Alfano, The Supercontinuum Laser Source (Springer-Verlag, New York, 1989). selective membrane for the analyte of inter- mixtures of metallic and semiconducting 8. J. K. Ranka, R. S. Windeler, A. J. Stentz, Opt. Lett. 25, est; binding of a charged species then results NTs, which make systematic studies difficult 25 (2000). in depletion or accumulation of carriers with- because metallic “devices” will not function 9. P. B. Corkum, N. H. Burnett, M. Y. Ivanov, Opt. Lett. 19, 1870 (1994). in the transistor structure (1, 2). An attractive as expected, and (ii) flexible methods for the 10. A. E. Kaplan, Phys. Rev. Lett. 73, 1243 (1994); A. V. feature of such chemically sensitive FETs is modification of NT surfaces, which are re- Sokolov, D. R. Walker, D. D. Yavuz, G. Y. Yin, S. E. that binding can be monitored by a direct quired to prepare interfaces selective for Harris, Phys. Rev. Lett. 85, 562 (2000). 11. R. A. Kaindl et al., J. Opt. Soc. Am. B 17, 2086 (2000). change in conductance or related electrical binding a wide range of analytes, are not well 12. R. W. Schoenlein et al., Science 274, 236 (1996). property, although the sensitivity and poten- established. 13. R. L. Byer, J. Mod. Opt., in press. tial for integration are limited. Nanowires of semiconductors such as Si 14. J. Ye, J. L. Hall, Opt. Lett. 24, 1838 (1999). The physical properties limiting sensor do not have these limitations, as they are 15. T. Udem, J. Reichert, R. Holzwarth, T. W. Ha¨nsch, Opt. Lett. 24, 881 (1999). devices fabricated in planar semiconductors always semiconducting, and the dopant type 16. ࿜࿜࿜࿜ , Phys. Rev. Lett. 82, 3568 (1999). can be readily overcome by exploiting and concentration can be controlled (7–9), 17. S. A. Diddams et al., Phys. Rev. Lett. 84, 5102 (2000). nanoscale FETs (5–9). First, binding to the which enables the sensistivity to be tuned in 18. J. Ye, J. L. Hall, S. A. Diddams, Opt. Lett. 25, 1675 (2000). surface of a nanowire (NW) or nanotube the absence of an external gate. In addition, it 19. L. Hollberg et al., IEEE J. Quantum Electron., in press. (NT) can lead to depletion or accumulation of should be possible to exploit the massive 20. D. J. Jones et al., Science 288, 635 (2000). carriers in the “bulk” of the nanometer diam- knowledge that exists for the chemical mod- 21. A. Apolonski et al., Phys. Rev. Lett. 85, 740 (2000). 22. H. R. Telle et al., Appl. Phys. B 69, 327 (1999). eter structure (versus only the surface region ification of oxide surfaces, for example, from 23. Supplemental material is available at Science Online of a planar device) and increase sensitivity to studies of silica (12) and planar chemical and at www.sciencemag.org/cgi/content/full/293/5533/ the point that single-molecule detection is biological sensors (4, 13), to create semicon- 1286/DC1 24. M. T. Asaki et al., Opt. Lett. 18, 977 (1993). possible. Second, the small size of NW and ductor NWs modified with receptors for 25. L. S. Ma, R. K. Shelton, H. Kapteyn, M. Murnane, J. Ye, NT building blocks and recent advances in many applications. Here we demonstrate the Phys. Rev. A, 64, 021802(R) (2001). assembly (9, 10) suggest that dense arrays of potential of NW nanosensors with direct, 26. S. A. Crooker, F. D. Betz, J. Levy, D. D. Awschalom, Rev. Sci. Instrum. 67, 2068 (1996). sensors could be prepared. Indeed, NT FETs highly sensitive real-time detection of chem- 27. D. W. Allan, Proc. IEEE 54, 221 (1966). were shown recently by Dai and co-workers ical and biological species in aqueous 28. D. N. Fittinghoff et al., Opt. Lett. 21, 884 (1996). to function as gas sensors (11). Calculations solution. 29. J.-C. Diels, W. Rudolph, Ultrashort Laser Pulse Phenome- na: Fundamentals, Techniques, and Applications on a The underlying concept of our experi- Femtosecond Time Scale (Academic Press, San Diego, CA, ments is illustrated first for the case of a pH 1996). 1Department of Chemistry and Chemical Biology, nanosensor (Fig. 1A). Here a silicon NW 2 30. We are indebted to D. Anderson for the loan of essential Harvard University, Cambridge, MA 02138, USA. Di- (SiNW) solid state FET, whose conductance equipment and S. T. Cundiff for critical comments. We vision of Engineering and Applied Sciences, Harvard also thank R. Bartels, T. Weinacht, and S. Backus for University, Cambridge, MA 02138 USA. is modulated by an applied gate, is trans- useful discussions. The work at JILA is supported by formed into a nanosensor by modifying the NIST, NASA, NSF, and the Research Corporation. *These authors contributed equally to the work. †To whom correspondence should be addressed. E- silicon oxide surface with 3-aminopropyltri- 19 April 2001; accepted 4 July 2001 mail: [email protected] ethoxysilane (APTES) to provide a surface

www.sciencemag.org SCIENCE VOL 293 17 AUGUST 2001 1289 R EPORTS that can undergo protonation and deprotona- can be attributed to an approximately linear rapidly to a constant value upon addition of a tion, where changes in the surface charge can change in the total surface charge density 250 nM streptavidin solution and that this chemically gate the SiNW. The single-crystal (versus pH) because of the combined acid conductance value is maintained after the boron-doped (p-type) SiNWs used in these and base behavior of both surface groups. To addition of pure buffer solution (Fig. 2B). studies were prepared by a nanocluster-me- support this point, we also carried out pH- The increase in conductance upon addition of diated vapor-liquid-solid growth method de- dependent measurements on unmodified streptavidin is consistent with binding of a scribed previously (7, 8, 14). Devices were (only ϪSiOH functionality) SiNWs (Fig. negatively charged species to the p-type fashioned by flow aligning (10) SiNWs on 1D). These conductance measurements show SiNW surface and the fact that streptavidin oxidized silicon substrates and then making a nonlinear pH dependence: The conductance (pI ϳ 5to6)(21) is negatively charged at the electrical contacts to the NW ends with elec- change is small at low pH (2 to 6) but large at pH of our measurements. The absence of a tron-beam lithrography (7, 8, 15). Linear cur- high pH range (6 to 9). Notably, these pH conductance decrease with addition of pure rent (I) versus voltage (V) behavior was ob- measurements on unmodified SiNWs are in buffer is also consistent with the small disso- ϳ Ϫ15 served for all of the devices studied, which excellent agreement with previous measure- ciation constant (Kd 10 M) and corre- shows that the SiNW-metal contacts are ohm- ments of the pH-dependent surface charge spondingly small dissociation rate for biotin- ic, and applied gate voltages produced repro- density derived from silica (19) (Fig. 1D). streptavidin (21). ducible and predictable (7, 8) changes in the To explore biomolecular sensors, we In addition, several control experiments I-V. In the solid state FET (insets, Fig. 1B), functionalized SiNWs with biotin (20) and were carried out to confirm that the observed the conductance (dI/dV) measured in air at studied the well-characterized ligand-recep- conductance changes are due to the specific V ϭ 0 as a function of time (15) was stable tor binding of biotin-streptavidin (Fig. 2A) binding of streptavidin to the biotin ligand. for a given gate voltage and showed a step- (21). Measurements show that the conduc- First, addition of a streptavidin solution to an wise increase with discrete changes of the tance of biotin-modified SiNWs increases unmodified SiNW did not produce a change gate voltage from 10 to –10 V; plots of conductance versus gate voltage were nearly linear. The response of the conductance of APTES-modified SiNWs (16) to changes in solution pH was evaluated by fabricating a cell consisting of a microfluidic channel formed between a poly(dimethylsiloxane) (PDMS) mold (10, 17) and the SiNW/sub- strate. We could carry out continuous flow or static experiments and could readily change pH. Measurements of conductance as a func- tion of time and solution pH (Fig. 1B) dem- onstrate that the NW conductance increases stepwise with discrete changes in pH from 2 to 9 and that the conductance is constant for a given pH; the changes in conductance are also reversible for increasing and/or decreas- ing pH. A typical plot of the conductance versus pH (Fig. 1C) shows that this pH de- pendence is linear over the pH 2 to 9 range and thus suggests that modified SiNWs could function as nanoscale pH sensors. In addition, we believe the uncertainty in slope of the conductance versus pH obtained from the different SiNW pH sensors studied to date, 100 Ϯ 20 nS/pH, is quite good and could be improved by placing further effort on control- ling reproducibility of the SiNW surface modification. These results can be understood by con- sidering the mixed surface functionality of the modified SiNWs. Covalently linking APTES to SiNW oxide surface results in a Ϫ surface terminating in both NH2 and Ϫ Fig. 1. NW nanosensor for pH detection. (A) Schematic illustrating the conversion of a NW FET into SiOH groups (Fig. 1A), which have differ- NW nanosensors for pH sensing. The NW is contacted with two electrodes, a source (S) and drain ent dissociation constants, pKa (12, 18, 19). (D), for measuring conductance. Zoom of the APTES-modified SiNW surface illustrating changes in Ϫ At low pH, the NH2 group is protonated to the surface charge state with pH. (B) Real-time detection of the conductance for an APTES- Ϫ ϩ NH3 (18) and acts as a positive gate, modified SiNW for pHs from 2 to 9; the pH values are indicated on the conductance plot. (inset, which depletes hole carriers in the p-type top) Plot of the time-dependent conductance of a SiNW FET as a function of the back-gate voltage. (inset, bottom) Field-emission scanning electron microscopy image of a typical SiNW device. (C) SiNW and decreases the conductance. At Ϯ Ϫ Ϫ Ϫ Plot of the conductance versus pH; the red points (error bars equal 1 SD) are experimental data, high pH, SiOH is deprotonated to SiO , and the dashed green line is linear fit through this data. (D) The conductance of unmodified SiNW which correspondingly causes an increase in (red) versus pH. The dashed green curve is a plot of the surface charge density for silica as a conductance. The observed linear response function of pH. [Adapted from (19)]

1290 17 AUGUST 2001 VOL 293 SCIENCE www.sciencemag.org R EPORTS in conductance (Fig. 2C). Second, addition of tion. The time scale for the increase in con- suggest that the linear response dynamic a streptavidin solution in which the biotin- ductance (seconds), which we associate with range can be extended by applying a back binding sites were blocked by reaction with 4 m-antibiotin dissociation, is consistent with gate to the nanowire, where gating can either equivalents of d-biotin produced essentially the reported (24) dissociation rate constant, increase or decrease the intrinsic binding no change in the conductance of biotin-mod- ϳ0.1 sϪ1, for this antibody-antigen system. constant. ified SiNWs (Fig. 2D). These controls show The decrease in conductance upon m-antibi- The concept of using NW FETs modified that there is little nonspecific binding of the otin addition indicates that a positively with receptors or ligands for specific detec- protein to either bare or biotin-modified charged species binds to and gates the SiNW tion can also be extended in many directions. SiNW surfaces. and is also consistent with the positive charge As a final example, we investigated sensing We also explored the sensitivity limits of of m-antibiotin at the pH 7 of our experi- of calcium ions (Ca2ϩ), which are important biotin-modified SiNWs nanosensors and find ments (25, 26). Control experiments demon- for activating biological processes such as that it is possible to detect streptavidin bind- strate that the observed results are due to muscle contraction, protein secretion, cell ing down to a concentration of at least 10 pM specific antibody binding to the surface anti- death, and development (27). We created a (Fig. 2E) (22). This detection level is substan- gen. First, unmodified SiNWs do not exhibit Ca2ϩ sensor by immobilizing calmodulin tially lower than the nanomolar range dem- a conductance change after adding m-antibi- onto SiNW surfaces. Data recorded from onstrated recently by stochastic sensing of otin (Fig. 3B). Second, addition of immuno- modified SiNW devices (Fig. 4A) showed a single molecules (23). In addition, a time- globulin G (IgG), which is not specific for drop in the conductance upon addition of a 25 dependent increase in the conductance can be biotin, does not result in a change in conduc- ␮MCa2ϩ solution and a subsequent increase resolved immediately after streptavidin addi- tance, whereas subsequent addition of m- tion at very low concentrations. We believe antibiotin solution produces a conductance that this behavior reflects contributions from drop, as observed in Fig. 3C. the forward binding rate and/or diffusion, We extended these reversible, real-time although future studies will be required to antibody detection experiments by monitor- resolve these contributions. ing the SiNW sensor conductance as a func- The above studies demonstrate that our tion of m-antibiotin concentration. We ob- NW nanosensors are capable of highly sensi- served a linear change in the conductance as tive and selective real-time detection of pro- a function of m-antibiotin concentration be- teins, although the essentially irreversible bi- low ϳ10 nM and saturation at higher values otin-streptavidin binding interaction pre- (Fig. 3D). There are several important facts cludes real-time monitoring of varying pro- that can be gleaned from this data. First, from tein concentrations. To explore this issue, we the linear regime, we estimate that the disso- Ϫ9 studied the reversible binding of monoclonal ciation constant, Kd, is on the order of 10 antibiotin (m-antibiotin) with biotin (24). M, which is in good agreement with the Time-dependent conductance measurements value, ϳ10Ϫ9 M, determined previously (25). made on biotin-modified SiNWs (Fig. 3A) Second, we can monitor protein concentra- exhibit a well-defined drop after addition of tion in real time, which could have important m-antibiotin solution (20) followed by an implications in basic research, for example, increase in the conductance to about the orig- monitoring protein expression, and medical inal value upon addition of pure buffer solu- diagnostics. Lastly, preliminary experiments

Fig. 2. Real-time detection of protein binding. (A) Schematic illustrating a bi- otin-modified SiNW (left) and subse- quent binding of streptavidin to the SiNW surface (right). The SiNW and streptavidin are drawn approximately to scale. (B) Plot of conductance versus time for a biotin-modified SiNW, where region 1 corresponds to buffer solution, region 2 corresponds to the addition of Fig. 3. Real-time detection of reversible protein 250 nM streptavidin, and region 3 cor- binding. (A) Plot of conductance versus time for responds to pure buffer solution. (C) a biotin-modified SiNW, where region 1 corre- Conductance versus time for an un- sponds to buffer solution, region 2 corresponds to the addition of ϳ3 ␮M m-antibiotin (460 modified SiNW; regions 1 and 2 are the ␮ same as in (B). (D) Conductance versus g/ml), and region 3 corresponds to flow of time for a biotin-modified SiNW, where pure buffer solution. (B) Conductance versus region 1 corresponds to buffer solution time for an unmodified SiNW; regions 1 and 2 and region 2 to the addition of a 250 are the same as in (A). (C) Conductance versus nM streptavidin solution that was pre- time for a biotin-modified SiNW, where region incubated with 4 equivalents d-biotin. 1 corresponds to buffer solution, region 2 cor- (E) Conductance versus time for a bi- responds to the addition of bovine IgG (200 ␮g/ml), and region 3 corresponds to addition of otin-modified SiNW, where region 1 ϳ ␮ ␮ corresponds to buffer solution, region 2 3 M m-antibiotin (460 g/ml). Arrows mark corresponds to the addition of 25 pM the points when the solutions were changed. streptavidin, and region 3 corresponds (D) Plot of the conductance change of a biotin- to pure buffer solution. Arrows mark modified SiNW versus m-antibiotin concentra- the points when solutions were tion; the dashed line is a linear fit to the four changed. low concentration data points. Error bars equal Ϯ 1SD.

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13. P. N. Bartlett, in Handbook of Chemical and Biological solution. The solutions used to probe biotin-strepta- Sensors, R. F. Taylor, J. S. Schultz, Eds. (IOP Publishing, vidin binding were 1 mM phosphate buffer (pH 9) Philadelphia, 1996), pp. 139–170. with 10 mM NaCl. The d-biotin–saturated streptavi- 14. Y. Cui, L. J. Lauhon, M. S. Gudiksen, J. Wang, C. M. din solution was prepared by adding four equivalents Lieber, Appl. Phys. Lett. 78, 2214 (2001). of d-biotin (Sigma) to one equivalent of streptavidin. 15. SiNWs with diameters of either 10 or 20 nm were All the solutions used in biotin and m-antibiotin suspended in ethanol and flow aligned on oxidized Si (Sigma) binding studies were 1 mM phosphate buffer substrates (1 to 10 ohm⅐cm, 600-nm oxide; Silicon (pH 7) with 5 mM NaCl. The parasitic conductance in Յ Sense), and contact leads (50 nm Al or Ti ϩ 100 nm these experiments, 5 nS, was substantially less than Au) were defined with electron-beam lithography. the sensor signal, 40 to 100 nS. The separation between contacts was typically 2 to 4 21. M. Wilchek, E. A. Bayer, Methods Enzymol. 184,49 ␮m. The conductance of SiNW devices as a function (1990). of time was determined directly with a computerized 22. In addition, the detection sensitivity can be changed apparatus with lock-in amplifier (Stanford Research, by the doping concentration and should enable sin- SR 830); a 31-Hz sine wave with 30-mV amplitude at gle-molecule detection at sufficiently low concentra- zero dc bias was used in most measurements. The tion. As an example, a single charge on the NW conductances of the SiNW devices were between 500 surface will be detected if it generates a sufficiently Ͼ and 2000 nS (resistance, 2 megohms to 500 kilohms). large local potential barrier ( 100 meV at room This relatively small range testifies to the good con- temperature) for electronic motion. Assuming that a ϳ trol of doping in our NWs. single charge is 1 nm away from a 20-nm-diameter 16. Surface-functionalized SiNW devices were prepared NW, the carrier concentration will most likely be ϳ ␮ by cleaning in an oxygen plasma (0.35 torr, 25 W lower than the order of 1000 electrons/ m (or a power for 20 s) to remove contaminants, immersion few electrons/nm) for detection, which translates ϫ 18 19 Ϫ3 in 1% ethanol solution of APTES (Aldrich) for 20 min, into 3 10 to 10 cm . rinsing with ethanol for three times, followed by 23. L. Movileanu, S. Howorka, O. Braha, H. Bayley, Nature heating at 120°C for 5 min. The different pH solu- Biotechnol. 18, 109 (2000). tions were made from 10 mM phosphate buffers with 24. R. C. Blake II, A. R. Pavlov, D. A. Blake, Anal. Biochem. 272, 123 (1999). 2ϩ 100 mM NaCl. Solutions were flowed through PDMS Fig. 4. Real-time detection of Ca ions. (A) microchannels (10, 17) (100- to 200-␮m width and 25. H. Bagci, F. Kohen, U. Kuscuoglu, E. A. Bayer, M. Plot of conductance versus time for a calmod- height) at a flow rate of 0.5 ml/hour. The parasitic Wilchek, FEBS Lett. 322, 47 (1993). ulin-terminated SiNW, where region 1 corre- conductance through the solution was a constant 26. Sequence analysis shows that the binding region of sponds to buffer solution, region 2 corresponds (about 10 nS) and less than the signal from the SiNW antibiotin (IgG1) is positively charged at pH 7 (24). ϩ to the addition of 25 ␮MCa2 solution, and (about 100 nS). The remaining domains of this large protein are region 3 corresponds to pure buffer solution. 17. D. C. Duffy, J. C. McDonald, O. J. A. Schueller, G. M. relatively distant from the SiNW and thus should Whitesides, Anal. Chem. 70, 4974 (1998). have little effect on SiNW conductance. (B) Conductance versus time for an unmodified 27. C. B. Klee, T. C. Vanaman, Adv. Protein Chem. 35, 213 SiNW; regions 1 and 2 are the same as in (A). 18. D. V. Vezenov, A. Noy, L. F. Rozsnyai, C. M. Lieber, J. Am. Chem. Soc. 119, 2006 (1997). (1982). Arrows mark the points when solutions were 28. We thank L. Lauhon, L. Chen, and Q. Cui for helpful 19. G. H. Bolt, J. Phys. Chem. 61, 1166 (1957). changed. Calmodulin-modified NWs were pre- discussion and T. Deng for technical assistance. C.M.L. pared by placing a drop (ϳ20 ␮l) of calmodulin 20. Biotin-modified SiNWs were prepared by depositing acknowledges support of this work by the Office of ␮ a drop (ϳ20 ␮l) of phosphate-buffered solution (PBS) solution (250 g/ml) on SiNW for 1 hour and ␮ Naval Research and the Defense Advanced Projects (250 g/ml; pH 5.6) solution of biotinamidocaproyl- Research Agency. then rinsing with water for three times. labeled bovine serum albumin (Sigma) on SiNWs for 2 hours, followed by a five times rinse with buffer 21 May 2001; accepted 23 July 2001 when a Ca2ϩ-free buffer was subsequently flowed through the device. Control experi- ments carried out with unmodified SiNWs (Fig. 4B) did not exhibit a conductance change when Ca2ϩ is added and thus demon- Stable Ordering in strate that the calmodulin receptor is essential for detection. In addition, the observed con- Langmuir-Blodgett Films ductance decrease in modified SiNWs is con- Dawn Y. Takamoto,1 Eray Aydil,1 Joseph A. Zasadzinski,1* sistent with expected chemical gating by pos- 2 2 3 itive Ca2ϩ, and the estimated dissociation Ani T. Ivanova, Daniel K. Schwartz, † Tinglu Yang, 3 constant, 10Ϫ5 to 10Ϫ6 M, is consistent with Paul S. Cremer

the reported Kd for calmodulin (27). Defects in the layering of Langmuir-Blodgett (LB) films can be eliminated by References and Notes depositing from the appropriate monolayer phase at the air-water interface. LB 1. P. Bergveld, IEEE Trans. Biomed. Eng. BME-19, 342 films deposited from the hexagonal phase of cadmium arachidate (CdA2)atpH (1972). 7 spontaneously transform into the bulk soap structure, a centrosymmetric 2. G. F. Blackburn, in Biosensors: Fundamentals and Ap- plications, A. P. F. Turner, I. Karube, G. S. Wilson, Eds. bilayer with an orthorhombic herringbone packing. A large wavelength folding (Oxford Univ. Press, Oxford, 1987), pp. 481–530. mechanism accelerates the conversion between the two structures, leading to 3. D. G. Hafeman, J. W. Parce, H. M. McConnell, Science a disruption of the desired layering. At pH Ͼ 8.5, though it is more difficult to 240, 1182 (1988). 4. F. Seker, K. Meeker, T. F. Kuech, A. B. Ellis, Chem. Rev. draw LB films, almost perfect layering is obtained due to the inability to convert 100, 2505 (2000). from the as-deposited structure to the equilibrium one. 5. S. J. Tans, A. R. M. Verschueren, C. Dekker, Nature 393, 49 (1999). Langmuir-Blodgett films are made by pulling zation is the progression from the as-depos- 6. P. G. Collins, M. S. Arnold, P. Avouris, Science 292, a substrate through a monolayer of amphiphi- ited structure to the thermodynamic equilib- 706 (2001). 7. Y. Cui, X. Duan, J. Hu, C. M. Lieber, J. Phys. Chem. B lic molecules at the air-water interface. Under rium structure. However, as Fig. 1B shows, 104, 5213 (2000). the appropriate conditions, the monolayer is the reorganization can be slowed to the point 8. Y. Cui, C. M. Lieber, Science 291, 851 (2001). transferred to the substrate (1, 2). Although that nearly perfect LB multilayer films can be 9. X. Duan, Y. Huang, Y. Cui, J. Wang, C. M. Lieber, Nature 409, 66 (2001). the LB technique has been used for decades, made by depositing from a different mono- 10. Y. Huang, X. Duan, Q. Wei, C. M. Lieber, Science 291, applications of the method have been frus- layer phase that exists at the air-water inter- 891 (2001). trated by defects ranging from pinholes to face at pH Ͼ 8.5. The high-pH monolayer 11. J. Kong et al., Science 287, 622 (2000). 12. R. K. Iler, The Chemistry of Silica ( Wiley, New York, larger scale reorganization of the layers (Fig. phase has a more condensed and lower ener- 1979). 1A) (3, 4). We show here that this reorgani- gy lattice structure than the monolayer at pH

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