Core−Shell Nanostructured Nanoparticle Films as Chemically

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Anal. Chem. 2001, 73, 4441-4449

Core-Shell Nanostructured Nanoparticle Films as Chemically Sensitive Interfaces Li Han, David R. Daniel, Mathew M. Maye, and Chuan-Jian Zhong*

Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902

This paper reports the results of an investigation of vapor molecule sorption at different types of nanostructured nanoparticle films. Core-shell nanoparticles of two different core sizes, Au2-nm and Au5-nm, and molecular linkers of two different binding properties, 1,9-nonanedithiol and 11-mercaptoundecanoic acid, are utilized as building blocks for constructing chemically sensitive interfaces. The work couples measurements of two different transducers, interdigitated microelectrodes and quartz crystal microbalance, to determine the correlation of the electronic resistance change and the mass loading with vapor sorption. The responses to vapor sorption at these nanostructured interfaces are demonstrated to be dependent on the core size of nanoparticles and the chemical nature of linking molecules. The difference of molecular interactions of vapor molecules at the nanostructured interface is shown to have a significant impact on the response profile and sensitivity. For the tested vapor molecules, while there are small differences for the sorption of nonpolar and hydrophobic vapor molecules, there are striking differences for the sorption of polar and hydrophilic vapor molecules at these nanostructured interfacial materials. The implication of the findings to the delineation of design parameters for constructing coreshell nanoparticle assemblies as chemically sensitive interfacial materials is also discussed. The study of nanostructured functional materials has attracted tremendous interest because of their potential utilities in microelectronics, catalysis, molecular recognition, and chemical and biological sensors.1-5 Recent advances involve organic monolayerencapsulated inorganic nanoscrystal cores,1-2,6-9 i.e., “core-shell” nanoparticles. The nanoscale functional properties of this type of nanoparticle are closely related to size, shape, and surface properties. A key challenge to the ultimate exploitation of this class of novel nanomaterials is the development of abilities to assemble the nanoparticles into nanostructured thin films with * To whom correspondence should be addressed. Phone: 607-777-4605. E-mail: [email protected]. (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27 and references therein. (2) Whetten, R. L.; Shafigulin, M. N.; Khoury, J. T.; Schaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (3) Link, S.; El-Sayed, M. A. Int. Rev. Phys. Chem. 2000, 19, 409. (4) Storhoff, J. J.; Mirkin, C. Chem. Rev. 1999, 99, 1849. (5) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. 10.1021/ac0104025 CCC: $20.00 Published on Web 08/10/2001

© 2001 American Chemical Society

predictable structural properties. A number of approaches to constructing functionalized nanoparticle and assemblies have been emerging.8-12 One important advance in the study of core-shell nanoparticle reactivities is the finding of place-exchange reactivity reported first by the Murray group.9 The shell reactivity enables the engineering of the shell structure with a desired composition or functionality. In the area of exploring nanoparticles for chemical sensing and molecular recognition, several recent studies have demonstrated intriguing functional properties involving nanostructure/ liquid and nanostructure/gas interfaces.7-9,11,13-19 In a report by Wohltjen and Snow,18 a chemiresistor sensor based on octanethi(7) (a) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906. (b) Wuelfing, W. P.; Templeton, A. C.; Hicks, J. F.; Murray, R. W. Anal. Chem. 1999, 71, 4069-4074. (c) Chen, S. W.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (d) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (e) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663. (f) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (g) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (8) (a) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (b) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C, J. Adv. Mater. 1995, 7, 795. (c) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (d) Brust, M.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. J. Am. Chem. Soc. 1998, 120, 12367. (e) Baum, T.; Bethell, D.; Brust, M.; Schiffrin, D. J. Langmuir 1999, 15, 866. (f) Horswell, S. L.; O’Neil, I. A.; Schiffrin, D. J. J. Phys. Chem. B. 2001, 105, 941. (g) Gittins, D. I.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67. (9) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (c) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (d) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 19061911. (e) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (10) (a) Musick, M. D.; Pena, D. J.; Botsko, S. L.; McEvoy, T. M.; Richardson, J. N.; Natan, M. J. Langmuir 1999, 15, 844. (b) Keating, C. D.; Musick, M. D.; Lyon, L. A.; Brown, K. R.; Baker, B. E.; Pena, D. J.; Feldheim, D. L.; Mallouk, T. E.; Natan, M. J. ACS Symp. Ser. 1997, No. 679, 7. (11) (a) Mirkin, C.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (b) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (12) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (13) (a) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514. (b) Templeton, A. C.; Zamborini, F. P.; Wuelfing, W. P.; Murray, R. W. Langmuir 2000, 16, 6682. (14) (a) Leibowitz, F. L.; Zheng, W. X.; Maye, M. M.; Zhong, C. J. Anal. Chem. 1999, 71, 5076. (b) Zheng, W. X.; Maye, M. M.; Leibowitz, F. L.; Zhong, C. J. Anal. Chem. 2000, 72, 2190. (c) Han, L.; Maye, M. M.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. J. Mater. Chem. 2001, 11, 1259. (15) Lyon, L. A.; Musick, M. D.; Natan, M. J. Anal. Chem. 1998, 70, 5177.

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olate-capped gold nanoclusters (∼2 nm) as films on an interdigitated microelectrode device was shown to exhibit remarkable electronic responses to organic vapor exposures. The nanoparticle films were prepared by spray deposition from a chloroform solution onto the electrode device held at a temperature of 120 °C and followed by solvent evaporation. In a similar approach,19 gold nanoparticles capped with aromatic thiol derivatives of different functional groups were cast on a microelectrode patterned surface and were exploited for influencing the relative strength of particle/particle and particle/solvent interactions, which was shown to impact the electronic properties upon exposure to different vapors. These results have demonstrated the viability of using monolayer-encapsulated gold nanoparticles as sensitive coatings. Such nanomaterials may be proven as a choice of advanced materials for developing novel chemical sensors. Other examples of developing thin-film materials as sensor coatings include polymer,20,21 polymer-carbon composite,22 and graphite microparticles.23 While solvent evaporation is straightforward in nanoparticle thin-film preparation, some of the challenging issues involve structural manipulation and stability. Upon vapor sorption, extensive structural rearrangement of the deposited particles due to weak hydrophobic interactions could easily occur to alter the thinfilm electronic properties. The emerging interests in nanoparticle assembly via molecularly defined chemical linkages and networking promise to address such issues. Importantly, the introduction of molecular linkages would permit structural controllability at the molecular level, which is evidenced by recent progress in the development of nanoparticle assembling strategies such as DNA linking,11 place exchange,9 and stepwise layer-by-layer construction.8,10,13 On the basis of the place-exchange reactivity of coreshell nanoparticles,9 we recently developed an effective nanoconstruction route that was termed one-step exchange-cross-linkingprecipitation route. This route was demonstrated to be applicable for assembling nanoparticle network thin films onto many types of substrate.14 Chart 1 illustrates two types of core-shell nanoparticle assembly derived from the one-step route. The nanostructures involve two different ω-functionalized thiol linkages, the covalent bonding at both ends of 1,9-nonanedithiolate (NDT) and the headto-head hydrogen bonding at the terminals of the gold-bound 11mercaptoundecanoic acid (MUA). Because of the simplicity of the film preparation and the possibility for structural tailoring of interparticle spatial and chemical properties, the nanoparticle (16) (a) Lahav, M.; Gabai, R.; Shipway, A. N.; Willner, I. Chem. Commun. 1999, 1937. (b) Lahav, M.; Shipway, A. N.; Willner, I. J. Chem. Soc., Perkin Trans. 1999, 2, 1925. (17) (a) Liu, J.; Alvarez, J.; Kaifer, A. E. Adv. Mater. 2000, 12, 1381. (b) Liu, J.; Mendoza, S.; Roman, E.; Lynn, M. J.; Xu, R. L.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304 (18) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (19) Evans, S. D.; Johnson, S. R.; Cheng, Y. L.; Shen, T. J. Mater. Chem. 2000, 10, 183. (20) Weimar, U.; Gopel, W. Sens. Actutors, B. 1998, 52, 143. (21) (a) Miller, L. L.; Kunugi, Y.; Canaves, A.; Rigaut, S.; Moorefield, C. N.; Newkome, G. R. Chem. Mater. 1998, 10, 1751. (b) Miller, L. L.; Boyd, D. C.; Schmidt, A. J.; Nitzkowski, S. C.; Rigaut, S. Chem. Mater. 2001, 13, 9. (22) (a) Severin, E. J.; Lewis, N. S. Anal. Chem. 2000, 72, 2008. (b) Sotzing, G. A.; Briglin, S. M.; Grubbs, R. H.; Lewis, N. S. Anal. Chem. 2000, 72, 3181. (23) (a) Shinar, R.; Liu, G.; Porter, M. D. Anal. Chem. 2000, 72, 5981. (b) Finklea, H. O.; Phillippi, M. A.; Lompert, E.; Grate, J. W. Anal. Chem. 1998, 70, 1268.

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Analytical Chemistry, Vol. 73, No. 18, September 15, 2001

Chart 1

assembly offers clear advantages over evaporation-prepared films as chemically sensitive interfaces. The nanostructured films are electronically conductive depending on core size and molecular linkage properties.14 There have been in-depth studies of the electronic conductivities of nanoparticle films formed by stepwise assembling or casting.8a,10a,24 In a recent report,24a it was shown that the electronic conductivity in solid-state films of alkanethiolate monolayer protected Au clusters occurs by a bimolecular, electron self-exchange reaction, whose rate constant is controlled by the core-to-core tunneling of electronic charge along alkanethiolate chains and the mixed valency of the nanoparticle cores. Our aim in this paper is to investigate the electronic resistance responses to interfacial vapor sorption at our nanostructured films. Our approach couples measurements of two different transducers, interdigitated microelectrodes (IME) and quartz crystal microbalance (QCM). Such measurements could probe the correlation of the electronic resistance change with vapor sorption for delineating design parameters of the nanostructured sensing interfaces. We note that similar resistance study coupled with QCM measurement was recently reported for exploring polymer-carbon composite films as sensor coatings for vapor sorption.22 EXPERIMENTAL SECTION Chemicals. Decanethiol (DT, 96%), hydrogen tetrachloroaurate (HAuCl4, 99%), tetraoctylammonium bromide (TOABr, 99%), 11-mercaptoundecanoic acid (MUA, 95%), and 1,9-nonanedithiol (NDT, 95%) were purchased from Aldrich and used as received. Other chemicals included hexane (Hx, 99.9%) and toluene (Tl, 99.8%) from Fisher, and methanol (Me, 99.9%) and ethanol (Et, 99.9%) from Aldrich. Water was purified with a Millipore Milli-Q water system. Synthesis. Gold nanoparticles of 2-nm core size (Au2-nm, 1.9 ( 0.7 nm) encapsulated with decanethiolate monolayer shells were (24) (a) Wuelfing, P. W.; Green, S. J.; Pietron, J. J.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 11465-11472. (b) Terrill, R. H.; Postlethwaite, T. A.; Chen, C. H.; Poon, C. D.; Terzis, A.; Chen, A. D.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537.

synthesized using the standard Schiffrin two-phase method6 and modifications for synthetic control.7f Details for the synthesis of our nanoparticles were recently described.14,25 Nanoparticles of Au5-nm (5.2 ( 0.3 nm) were derived from the above Au2-nm particles by a thermally activated processing route recently developed in our laboratory.14,25 The core-shell structure, particle core size, shape morphology, and crystalline properties were characterized by a number of techniques, including FT-IR, transmission electron microscopy, X-ray diffraction, electron diffraction, and UV-visible spectrophotometry.25 Preparation. The nanoparticle thin films prepared for the present work included two types: (1) NDT-linked nanoparticles (NDT-Au2-nm or -Au5-nm), and (2) MUA-linked nanoparticles (MUA-Au2-nm or -Au5-nm). The thin films were prepared via an “exchanging-cross-linking-precipitation” route, as detailed in our recent reports.14 The reaction involved an exchange of NDT or MUA with the gold-bound alkanethiolates, followed by crosslinking and precipitation via either Au-S bonding at both ends of NDT or hydrogen bonding at the carboxylic acid terminals of MUA. For MUA-Aunm films, stock solutions of DT-capped Au5-nm (∼4 µM) or Au2-nm (∼120 µM) in hexane and MUA (5.0 mM) in ethanol were used. The MUA thiols were mixed with Aunm in a hexane solvent at a controlled concentration or ratio, typically in the concentration ranges of 0.5-2.0 µM for Au5-nm, 1.0-10.0 µM for Au2-nm, and 0.05-0.2 mM for MUA. Typical MUA to nanoparticle ratios were 50-500. Similarly for NDT-Aunm films, Au5-nm or Au2-nm particles and NDT dithiols were mixed in hexane. The typical concentration ranges were 0.5-2.0 µM for Au5-nm, 1-50 µM for Au2-nm, and 1-20 mM for NDT. Typical ratios of NDT to particle were 103-104. The gold-coated IME and QCM devices were precleaned by immersion in 1:3 H2O2 (30%)-H2SO4 (concentrated) solution and thorough rinsing in deionized water and ethanol. (Caution: the H2O2-H2SO4 solution reacts violently with organic compounds and should be handled with extreme care). The IME or QCM substrate was then immersed into the solution of the mixed nanoparticles and thiols at room temperature, and solvent evaporation was prevented during the film formation. The thickness of the thin films grown on the surface of the substrates was controlled by immersion time (3-60 h, depending on particle size and molecular linker14c). The thin films thus produced were thoroughly rinsed with the solvent and dried under nitrogen. Instrumentation and Measurements. Resistance and conductivity measurements were performed using an IME (Microsensor Inc.) as the substrate for thin-film deposition. The IME has 50 pairs of gold electrodes of 15-µm width and 15-µm spacing on quartz substrate (0.20 µm thick). A computer-interfaced electrical multimeter (Extech) was used to measure the lateral resistance of the film deposited on the IME. When the film is thinner than the finger thickness, the film resistance (RΩ) on the IME is related to the lateral electronic conductivity (σ) of the film by the relationship of σ ) (1/RΩ)(w/dL), where w is the gap width of the array electrodes, L is the length of the electrodes, and d is the film thickness. There was a recent in-depth study of the electronic conductivity of cast nanoparticle films.24a Our work reported herein focuses on the measurement of resistance (R) and relative differential resistance change ∆R/Ri for the evaluation

of the vapor sorption responses on thin films of thickness below or close to the finger thickness. ∆R is the difference of the maximum and minimum values in the resistance response, and Ri is the initial resistance of the film.22 QCM measurements were carried out using a home-built oscillation circuit with sine wave output (oscillator model MC12061L, Newark Electronics). A HP frequency counter (model 5302A) was used to measure the frequency. AT-cut and polished quartz crystals with a 9-MHz fundamental resonance frequency were used. Gold films were deposited onto both sides of the quartz disk, which has an excitation electrode diameter of 0.6 cm. The determination of the frequency change for the QCM transducer before and after a film is deposited provides information on the mass loading of the nanoparticle film. Under pure mass effect, the frequency change (∆f, Hz) can be related to mass change (∆m, g/cm2) by the Sauerbrey’s equation,26

(25) (a) Maye, M. M.; Zheng, W. X.; Leibowitz, F. L.; Ly, N. K.; Zhong, C. J. Langmuir 2000, 16, 490. (b) Maye, M. M.; Zhong, C. J. J. Mater. Chem. 2000, 10, 1895.

(26) (a) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (b) Applications of Piezoelectric Quartz Crystal Microbalances; Lu, C., Canderna, A. W., Eds.; Elsevier: Amsterdam, 1984.

∆m ) -Cf∆f

(1)

where the mass sensitivity Cf ) 5.5 ng cm-2 Hz-1 (9-MHz QCM). While it is possible that the film thickness on IME and on QCM is different (QCM has a gold surface, and IME has 50% gold and 50% quartz surfaces), our recent AFM data showed that the difference was rather small ((10%) between gold and quartz substrates for a film prepared by an immersion time of ∼20 h. In addition, similar data were obtained for film slightly thinner or slightly thicker on the IME than on QCM, which were realized by controlling the immersion time.14c Both the IME and the QCM devices were housed in a Teflon chamber (2 × 2 × 2 cm3) with tubing connections to vapor and N2 sources; the electrode leads were connected to the multimeter and the oscillator, respectively. The resistance and frequency measurements were performed simultaneously with computer control. All experiments were performed at room temperature, 22 ( 1 °C. The partial vapor pressures (with respect to atmospheric pressure) for hexane, toluene, water, methanol, ethanol, propanol, and butanol at this temperature are 0.1826, 0.0333, 0.0296, 0.1510, 0.0730, 0.0218, and 0.0082, respectively. N2 gas (99.99%, Progas) was used as reference gas and as diluent to change vapor concentration by controlling mixing ratio. The gas flow was controlled by a calibrated Aalborg mass-flow controller (AFC-2600). The flow rates of the vapor stream were varied between 5 and 50 mL/min, with N2 added to a total of 100 mL/ min. The vapor-generating system followed the standard protocol.23a The vapor stream was produced by bubbling dry N2 gas through a bubbler of the vapor solvent using the controller to manipulate vapor concentration. The ppm concentration was calculated from the partial vapor pressure and the mixing ratio. The bubbler had a headspace for equilibrating the vapor and was connected to an inlet for admitting N2 and an outlet for delivering vapor. In the experiment, the reference N2 was passed through the test fixture to establish the baseline before and after each vapor exposure. RESULTS AND DISCUSSION The film formation and structural properties of our molecularly linked nanocrystal films have recently been characterized by

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Table 1. Estimates of Surface Density, Γ1-layer, Thickness, d1-layer, and Frequency Change, ∆f1-layer, for One Equivalent Layer of Nanoparticles Derived from Different Core Sizes and Different Shell Linkages film

Γ1-layer (g/cm2)a

d1-layer (nm)b

∆f1-layer (Hz)

MUA-Au5-nm NDT-Au5-nm MUA-Au2-nm NDT-Au2-nm

3.0 × 10-6 4.8 × 10-6 4.0 × 10-7 8.7 × 10-7

8.7 6.5 5.6 3.5

545 867 73 158

a Assuming (100) packing model. b Interparticle distance: 1.5 nm for NDT; 3.6 nm for (MUA)2.

Table 2. Estimates of ∆ffilm, Nfilm, and dfilm for Thin Films of Nanoparticles Experimentally Prepared from Different Core Sizes and Different Shell Linkages film

R (kΩ)

∆ffilm (Hz)

Nfilm (layers)a

dfilm (nm)

MUA-Au5-nm NDT-Au5-nm MUA-Au2-nm NDT-Au2-nm

337.9 1.2 3800.0 556.1

9875 10283 5194 9863

9 6 36 31

78 39 202 109

a

N ) (∆ffilm/∆f1-layer)/2.

QCM, UV-visible, electrochemical, transmission electron microscopic (TEM), atomic force microscopic (AFM), and infrared reflection spectroscopic (IRS) techniques.14 In these studies, we have demonstrated that the film thickness can be controlled by formation time. The thickness was estimated based on QCMdetermined frequency change and a truncoctahedron model for Aunm.7f For average core sizes of 2.0 and 5.1 nm, the number of gold atoms and thiolate chains are 225 (Au) and 71 (RS) and 4794 (Au) and 506 (RS), respectively.7f Table 1 shows the estimated values for one equivalent nanoparticle layer on a planar substrate for nanoparticle assemblies derived from various combinations of MUA, NDT, Au2-nm, and Au5-nm. The film thickness and the equivalent number of layers (N) thus estimated are listed in Table 2 for several thin films prepared for characterizations in this work. The film thickness ranged from 40 to 200 nm, depending on the molecular linkage and the nanocrystal core size. The resistance values are included for comparison; the order of magnitude difference reflects the difference in conductivity for these films.14a,b An increase in thickness was shown to decrease the resistance. The differences of the formation kinetics for these films were recently studied using both UV-visible and QCM.14 In our previous studies,14 UV-visible data demonstrated that there was a small red shift for the SP band, which was largely attributed to interparticle distance change in the film due to the molecular linkage. AFM data demonstrated the individually isolated nanoparticle character in these nanostructured films. IRS data showed that the shell DT were largely replaced by the molecular linkers, NDT or MUA, as evidenced by the detection of reduced or diminished methyl band intensities in the diagnostic C-H region and bands in the carbonyl region for the MUA-linked films. The COOH linkages were characteristic of head-to-head hydrogen bonding, which could further be tuned by pH14b or derivatized to anhydride.27 4444 Analytical Chemistry, Vol. 73, No. 18, September 15, 2001

Studies of electronic conductivity properties of gold nanoparticle films prepared by casting and stepwise-assembling methods have recently been reported by a number of researchers.8a,10a,24 The conductivity is known to be dependent on core size and shell chain length.24 Our preliminary work14a,b indicated that the conductivity values for our NDT-linked Au5-nm and Au2-nm films were ∼10-1 and ∼10-3 S/cm, respectively, whereas those for our MUA-linked Au5-nm and Au2-nm films were ∼10-4 and ∼10-6 S/cm, respectively. While the data are qualitatively consistent with the previous conclusions on dependencies on chain length and core size, a precise assessment of the electronic conductivity for our films is yet to be systematically studied. Since the present paper deals only with conductivity change, we will analyze exclusively the resistance data to evaluate the electronic responses upon vapor sorption. On the basis of studies of electronic conductivity for the coreshell nanoparticle films,24 core size and shell linkage are expected to be key parameters for manipulating the vapor response. Films with larger sized cores (e.g., Au5-nm) are known to be more conductive than smaller sized cores (Au2-nm), whereas films with a larger interparticle separation distance (e.g., MUA) are less conductive than those with a smaller separation distance (e.g., NDT). Such electronic properties are expected to change upon partition of vapor molecules into the nanostructures. We have examined a number of small organic molecules as vapor probes with differences in hydrophobicity, polarizability, polarity, and hydrogen-bonding properties, including hexane, toluene, methanol, ethanol, and water. The next two subsections present the results separately in terms of particle core size, i.e., Au5-nm and Au2-nm. 1. Au5-nm Films. A typical set of response curves for hexane (A) and toluene (B) vapor sorption at a MUA-Au5-nm film is shown in Figure 1. The resistance (R) and frequency change (∆f) were simultaneously measured as a function of time during vapor exposures of different concentrations (Cppm). The baseline corresponds to 100% nitrogen flow. The resistance increased upon introduction of vapor (“on”), and reversed upon nitrogen flush (“off”). The reversible response pattern corresponded to sorption and desorption; the magnitude of the response increased with increasing Cppm. The baseline was largely maintained by flushing with nitrogen. In comparison with the resistance response, the decrease of frequency provided a measure of the amount of vapor molecules sorbed into the film. The sorbed mass increased with vapor concentration. The mirror-type image of the two sets of data was expected in view of the difference in signal transduction. A noticeable noise (∼10%) in the frequency data is due to noise from our current frequency counter. Similar response profiles were also observed for other vapor probes such as methanol and water, but in much smaller magnitudes. Qualitatively, the increase in resistance was accompanied by sorption of vapor molecules into the film. Quantitatively, the frequency change can be converted to mass change for the sorbed vapor molecules by eq 1. A frequency change of 60 Hz, which is a typical response to hexane vapor of 1500 ppm, translates to a mass change of 330 ng/cm2. Using a (27) Zhang, F. X.; Zheng, W. X.; Maye, M. M.; Lou, Y. B.; Han, L.; Zhong, C. J. Langmuir 2000, 16, 9639.

Figure 1. Response curves of resistance (R) and frequency change (∆f) for toluene (top) and hexane (bottom) vapor sorption on MUAAu5-nm film (N ≈ 9). Vapor concentrations (from left to right): 137, 274, 410, and 547 ppm for toluene; 749, 1499, 2248, and 2998 ppm for hexane.

diameter of the averaged hexane molecular length or width,28 we roughly estimate one equivalent layer of hexane on a planar surface to be about 5.5 × 10-10 mol/cm2, or 47 ng/cm2, which translates to 9 Hz. The 60-Hz change corresponds roughly to about seven equivalent layers of hexane molecules. Up to ∼20 equivalent layers of hexane sorption have been observed at higher vapor concentrations, which is reasonable in view of the bulk adsorption property in the nanoporous nanoparticle film.14b To compare the response sensitivity of different vapor molecules at the MUA-Au5-nm film, we plot the relative differential resistance change ∆R/Ri versus Cppm (A) and ∆f versus Cppm (B) for four different vapor molecules in Figure 2. We note that the evaluation of response sensitivity based on ∆R/Ri was reported for studies of vapor sorption on polymer-carbon composite films.22 Such normalization corrects the difference in film resistance when we compare responses of different films. Clearly, the dependence of both the differential resistance and the frequency change on vapor concentration is approximately linear for the four different vapor probes in the tested concentration ranges. The observation of linear responses, which is similar to vapor sorption studies reported for other films such as polymer-carbon composite22 and graphite microparticles,23 is suggestive of a bulk adsorption phenomenon at the thin films below saturation, which could be described by Henry’s law for bulk partitioning.23b The possible existence of nonequivalent adsorption sites in the nanostructured film could also complicate a description simply based on ideal models such as Langmuir adsorption isotherm or Henry’s law for (28) Richards, R. E.; Rees, L. V. C. Langmuir 1987, 3, 335.

Figure 2. Relative differential resistance change, ∆R/Ri (A), and frequency change, ∆f (B), vs vapor concentration, Cppm, for MUAAu5-nm film (N ≈ 9). Inset: an enlarged view of the methanol and water data in the low-concentration region. Vapors: toluene (2), hexane (b), methanol (9), and water ([).

the entire concentration range. We note that in our linear regression of these data we did not force the data through the origin. It thus appeared that a finite intercept in the y-axis was apparent for some of the vapors if the regression was extended to the axis. Such a phenomenon has in fact been observed for other vapor sorption response systems.22a,23 The observation for our films suggests that both bulk and surface adsorption phenomena are likely operative, depending on the concentration range and the vapor properties, which are under a further investigation. The slopes, as listed in Table 3, provide a measure of the response sensitivity. The results display a decreased response sensitivity in the order of toluene > hexane > methanol and water. If we define the relative sensitivity (RS) ratio of one probe versus another,

RSR or f )

[

]

sensitivity(1) sensitivity(2)

(2)

resistance or frequency

the RS value provides an assessment of the difference in relative sensitivity. Interestingly, the RS values of toluene versus hexane response are remarkably comparable between the resistance data (RSR ) 5.4) and the frequency data (RSf ) 5.7). In this case, the hydrophobic and nonpolarizable hexane can be viewed as an internal standard. The result indicates that the vapor-induced resistance change can be directly related to mass loading of the sorbed molecules. The resistance increase is suggestive of the existence of vapor-induced increase of interparticle distance. This could be a result of a thin-film swelling effect, as encountered in polymer-carbon composite systems upon vapor sorption.22 We are currently using AFM to detect such an effect. The larger response sensitivity for toluene over hexane is believed to be associated with a strong interaction in the MUAAnalytical Chemistry, Vol. 73, No. 18, September 15, 2001

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Table 3. Response Sensitivity Data (i.e., Slopes from Figures 2-5) for Different Vapor Sorption at Thin Films of Different Core Sizes and Different Shell Linkages responses for different core sizes Au5-nm shell linkage MUA

NDT

a

vapor Tl Hx Me Wa Tl Hx Me Wa

(∆R/Ri)/∆C 10-3

2.24 × 4.17 × 10-4 7.04 × 10-5 2.96 × 10-5 2.71 × 10-4 (5.23 × 10-5)a 1.89 × 10-5 1.11 × 10-5

Au2-nm ∆f/∆C -0.24 -0.042 -0.010 -0.014 -0.18 -0.028

(∆R/Ri)/∆C 10-4

8.28 × 1.87 × 10-4 b b 3.14 × 10-4 (5.49 × 10-5)a b b

∆f/∆C -0.23 -0.030 -0.24 -0.036

Slopes from linear approximation in Figures 3 and 5. b No entry due to unusual response profiles (see Figure 6).

Figure 3. ∆R/Ri (A) and ∆f (B) vs Cppm for NDT-Au5-nm film (N ≈ 6). Inset: an enlarged view of the methanol and water data in the low-concentration region. Vapors: toluene (2), hexane (b), methanol (9), and water ([).

Au5-nm film, as supported by a close examination of the resistance and the frequency change kinetics between these two vapor molecules on the same film (Figure 1). The sorption-desorption response time is much slower for toluene (∼400 s) than for hexane (∼50 s). The high affinity of the film to hydrophobic molecules is evidenced by the large difference in RS ratios of hydrophobic versus hydrophilic molecules (e.g., RSR ) 32 and RSf ) 24 for toluene versus methanol). Between methanol and water, there is a small difference in the resistance sensitivity data (RSR ) 2.4 for methanol versus water), but not much in the frequency sensitivity data. The partial hydrophobic character of methanol is likely to play a role, which will be further discussed in a latter subsection. Similar response profiles have been observed for vapor sorption at a NDT-Au5-nm film. Figure 3 shows the ∆R/Ri-Cppm (A) and ∆f-Cppm (B) plots. Only the data for toluene and hexane are shown in Figure 3B, because frequency changes for methanol and water were too small to be significant due to the limitation of signal-to4446 Analytical Chemistry, Vol. 73, No. 18, September 15, 2001

noise ratio. We also note that the ∆R/Ri versus C plot for hexane (solid line) showed a deviation from linearity (dashed line). The nonlinearity implies the existence of a saturation effect for hexane sorption at the NDT-Au5-nm film. For the convenience of an overall assessment, we used the linear approximation for hexane sorption data. Most plots showed linear relationships as observed for the MUA-Au5-nm film. Again, the existence of nonlinearity and finite intercepts (if the regression is extended to axis) for some of the vapors is suggestive of the complication due to both bulk and surface adsorption phenomena. The results, as shown in Table 3, exhibit comparable sensitivity ratios between resistance and frequency data (e.g., RSR ) 5.2 and RSf ) 6.4 for toluene versus hexane). Other results include RSR ) 14 for toluene versus methanol, and RSR ) 1.7 for methanol versus water. In general, the response data for toluene and hexane sorption at NDT-Au5-nm film exhibit trends similar to those observed for the MUA-Au5-nm film. Major differences can be identified when we compare the sensitivity data between the MUA-Au5-nm and the NDT-Au5-nm films. For both hexane and toluene, the resistance sensitivity of MUA-Au5-nm film is ∼8 times larger than those for the NDT-Au5-nm films (RSR ) 8.0). In contrast, the corresponding frequency sensitivity values are almost comparable between these two films (RSf ) 1.3). These two films differ in thickness (Table 2) by a factor of ∼2. This observation suggests that the correlation between electronic conductivity and mass loading is dependent on the nature of the interparticle molecular linking properties. The above findings demonstrate that the nature of the shell or network structure has an impact on the resistance response sensitivity to vapor sorption. This is intriguing in view of the fact that the same amount of vapor sorption at the nanostructured thin films, as demonstrated by the frequency response data, could lead to dramatically different electronic responses. 2. Au2-nm Films. When a smaller particle core size, Au2-nm, was used for constructing the films, both similarities and differences were observed. Figures 4 and 5 present two representative sets of ∆R/Ri-Cppm (A) and ∆f-Cppm (B) plots for vapor sorption at MUA-Au2-nm film and NDT-Au2-nm films, respectively. Here only data for toluene and hexane are shown. For these two vapor molecules, both films exhibited response profiles similar to those described earlier for the Au5-nm films. The data for methanol and water vapor sorption will be discussed later in view of their unique characteristics of response profile. In the above plots, the

Figure 4. ∆R/Ri (A) and ∆f (B) vs Cppm for MUA-Au2-nm film (N ≈ 36). Vapors: toluene (2) and hexane (b).

Figure 5. ∆R/Ri (A) and ∆f (B) vs Cppm for NDT-Au2-nm film (N ≈ 31). Vapors: toluene (2) and hexane (b).

concentration dependence is basically linear for both resistance and frequency changes. Note that in the resistance plot for hexane at the NDT-Au2-nm film (Figure 5A) there is a subtle hint of a curved characteristic (nonlinearity) similar to the data for the NDT-Au5-nm film (Figure 3A). The existence of a finite intercept is again apparent if the regression is extended to the axis. For the MUA-Au2-nm film, while the qualitative trend is similar, the relative sensitivity ratio of toluene response versus hexane response is slightly smaller in the resistance data (RSR ) 4.4) than in the frequency data (RSf)7.7). Similarly, there is a small difference for the NDT-Au2-nm film (RSR ) 5.7 and RSf ) 6.7).

A comparison of the data between NDT-Au2-nm (Figure 5) and MUA-Au2-nm (Figure 4) films shows a subtle difference in relative sensitivity ratios of toluene versus hexane (Table 3). While the relative sensitivity ratio is slightly smaller for the MUA-Au2-nm film (RSR ) 4.4) than for the NDT-Au2-nm film (RSR ) 5.7), the relative ratio in frequency sensitivity is slightly larger (RSR ) 7.7 for MUA-Au2-nm and RSR ) 6.7 for NDT-Au2-nm). We believe that the differences and similarities in the response sensitivity to vapor sorption found for thin films of different molecular linkers and different core sizes, as summarized in Table 3, reflect the differences and similarities of several structural properties, including molecular interactions in terms of hydrophobicity, polarizability, polarity and solubility of the vapor molecules, interparticle binding and spatial properties defined by the chain length, the different linkage, and the core size. For example, while both toluene and hexane are hydrophobic and nonpolar molecules, toluene is more polarizable due to its π-electron character. This difference is consistent with the subtle difference in their dielectric constants ( ) 2.4 for toluene and  ) 1.9 for hexane22). Overall, for hydrophobic and nonpolar vapors such as toluene and hexane, the response differences are relatively small between the different thin films. For the polar and hydrophilic vapors such as methanol and water, the response differences are relatively large. Further details of the latter are discussed next. Having discussed the vapor sorption for the nonpolar and hydrophobic analytes, we now discuss the sorption data for polar analytes. Unusual response profiles were observed for polar vapor probes (e.g., water ( ) 78) and methanol ( ) 33)) for both NDTAu2-nm and MUA-Au2 nm films. Figure 6 shows two representative sets of resistance response data for NDT-Au2-nm (A) and MUAAu2-nm (B) films. For NDT-Au2-nm film (A), the sorption of both water (a) or methanol (b) vapor molecules exhibits spikelike responses. This is particularly evident for methanol sorption. A sharp rise of resistance is followed by a sharp return upon vapor “on”, and a sharp decrease is followed by a sharp return upon vapor “off”. The response characteristic is essentially indicative of a transient change of the conductivity during the initial stages of the vapor sorption and desorption. In contrast, for MUA-Au2-nm film (B), the vapor sorption shows a net negative response for both water and methanol vapor sorption. The data suggest that the sorption of these vapor molecules leads to an increase in conductivity of the film. The above response profiles are in striking contrast to the usual resistance increase response profile observed for the Au5-nm films of both molecular linkers (see Figure 1). This net negative response is intriguing for both fundamental and practical reasons. Fundamentally, the response profile serves as another way to assess the electronic conductivity of the nanostructured films. Practically, it provides another means to improve selectivity and sensitivity of sensor films. We note that a similar negative response profile for methanol sorption was reported recently for evaporated thin films of gold nanoparticles capped with aromatic thiol derivatives of different functional groups.19 In view of the partial hydrophobic character of methanol in comparison with water molecule, we further examined the response of the MUA-Au2-nm film to alcohols of longer chain length such as ethanol, propanol, and butanol. The increase of chain length increases the hydrophobicity. The data for ethanol (c) is shown in Figure 6B. The response profile for ethanol vapor Analytical Chemistry, Vol. 73, No. 18, September 15, 2001

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Figure 6. Response curves of resistance change (R) for vapor sorption of methanol and water on NDT-Au2-nm film (N ≈ 47) (A) and MUA-Au2-nm film (N ≈ 31) (B). The baselines are offset for comparison. Vapor concentrations: 142, 284, and 425 ppm for water (A, a); 620, 1239, 1859, and 2478 ppm for methanol (A, b); 142, 284, and 425 ppm for water (B, a); 620, 1239, and 1859 ppm for methanol (B, b); and 303, 606, and 909 ppm for ethanol (B, c).

displays a positive change in resistance. Evidently, the increase of the hydrophobic character reversed the response to the usual positive profile. Data for propanol and butanol showed the same trend and increased sensitivity. In Figure 7, the plots of ∆R/Ri-Cppm and ∆f-Cppm for several different polar vapor molecules (water, methanol, ethanol) are compared. These plots exhibit an approximately linear relationship for the resistance and frequency changes as a function of the vapor concentration. Clearly, the response sensitivity shows negative signs for both water and methanol vapors and a positive sign for ethanol vapor. The positive response sensitivity was found to further increase with increasing alkyl chain length, as observed for propanol and butanol (not shown). The striking difference of the response sensitivity to vapor sorption demonstrates the high sensitivity of interfacial vapor sorption to the molecular interactions and the detailed nanostructures. We are currently investigating several possible factors. These include the formation of H-bonding between the polar vapor molecule and the head-to-head H-bonded MUA linkage, the dielectric medium effect in the nanostructured film partitioned with the vapor molecules, and the solubility effect of the nanostructured film in alcoholic solvents. The latter could lead to rearrangement of the nanostructures. The fact that the negative response profile was observed only for films derived from smaller core size and longer molecular linker may be an indication of the sensitivity of the conductivity to the manipulation of interparticle regidity. We also examined the vapor sorption dependence on thin-film thickness to establish that the above observations are not due to 4448

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Figure 7. ∆R/Ri (A) and ∆f (B) vs Cppm for vapor sorption of water (a), methanol (b), and ethanol (c) at MUA-Au2-nm film. Vapors: water (b), methanol (9), and ethanol (2). Table 4. Response Sensitivities for Hexane Sorption at NDT-Au2-nm Films of Different Thicknesses response sensitivity equiv no. of layers

∆R/Ri vs C

∆f vs C

7.0 7.4 9.0 16.3 22.2

4.0 × 10-5 4.8 × 10-5 3.9 × 10-5 5.2 × 10-5 5.2 × 10-5

9.3 × 10-3 9.2 × 10-3 2.4 × 10-2 2.4 × 10-2 2.4 × 10-2

difference in film thickness. Hexane was used as a probe largely because of its simple hydrophobic or van der Waals interaction. Response sensitivities, as shown in Table 4, were obtained from measurements of the resistance and the frequency changes as a function of vapor concentration for NDT-Au2-nm films of different thickness. While the frequency sensitivity exhibited an increase (by a factor of 2.5) from 7 to ∼15 layers, the resistance sensitivity showed a small variation ((4.6 ( 0.6) × 10-5). Clearly, the data did not support a definite trend. The small changes indicated that the vapor penetration reached its limit at about eight equivalent layers. The result suggests that the thickness effect on resistance change sensitivity of IME to vapor sorption is relatively insignificant once the film reaches a certain thickness. Similar conclusions have also been reached for the other thin films. CONCLUSIONS The molecularly linked nanoparticle thin-film assemblies from two different molecular linkages (covalent NDT, hydrogenbonding MUA) and two different core sizes (2- and 5-nm Au) have been demonstrated to be viable nanostructured materials for constructing chemically sensitive interfaces. The physical or chemical nature of the interactions of vapor molecules at the nanostructured interface and the associated changes in interpar-

ticle structural properties are believed to have a significant impact on the response profile and sensitivity. A variety of molecular interactions may be operative, including hydrophobicity, polarity, and hydrogen bonding at the nanostructured films. The response profile is sensitive to both core size and shell structure, depending on the structural properties of the vapor molecules. For the tested vapors, while there are small differences for the sorption of nonpolar and hydrophobic vapor molecules, there are striking differences for the sorption of polar and hydrophilic vapor molecules on these nanostructured interfacial materials. These findings have important implications for the design of core-shell nanostructures as chemically sensitive interfaces for probing both fundamental and practical issues of interfacial molecular interaction and recognition. Further spectroscopic and microscopic

investigations of these interactions are underway to gain insights into the manipulation and control of the nanostructured chemical sensing properties. ACKNOWLEDGMENT Financial support of this work is gratefully acknowledged in part from the ACS Petroleum Research Fund and in part from the 3M fund. Preliminary work by W. X. Zheng and F. L. Leibowitz is also acknowledged.

Received for review April 6, 2001. Accepted June 28, 2001. AC0104025

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