Designing Thin Film-Capped Metallic Nanoparticles Configurations for

Article pubs.acs.org/JPCC

Designing Thin Film-Capped Metallic Nanoparticles Configurations for Sensing Applications Muhammad Y. Bashouti,*,†,‡ Adi-Solomon de la Zerda,‡ Dolev Geva,‡ and Hossam Haick‡ †

Max-Planck Institute for the Science of Light, Günther-Scharowsky-Strasse 1, 91058 Erlangen, Germany The Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion−Israel Institute of Technology, Haifa 32000, Israel

‡

S Supporting Information *

ABSTRACT: Thin film-capped metallic nanoparticles (TFCMNPs) hold big promise for rapid, low-cost, and portable tracing of gas analytes. We show that sensing properties can be controlled by the configuration of the TFCMNPs. To this end, two methods were developed: layer by layer (LbL) and drop-by-drop, i.e., drop casting (DC). The TFCMNP prepared via LbL method was homogeneous and gradually increased in thickness, absorbance, and conductivity relative to TFCMNP prepared via DC method. However, our results indicate that the sensing of TFCMNP devices prepared via DC is significantly higher than that of equivalent LbL devices. These discrepancies can be explained as follows: LbL forms a high dense layer of TFCMNPs without vacancies, and a well-controlled deposition of NPs. The distance between the adjacent NPs is controlled by the capped ligands and the linker molecules making a rigid TFCMNP. Thus, exposing LbL devices to analyte induces a marginal change in the NP−NP distance. However, in DC devices, the analyte induces major change in the NP distances and permittivity due to their lack of connection, making the sensing much more pronounced. The DC and LbL methods used thiol and amine ligands-capped metallic nanoparticles to demonstrate the applicability of the methods to all types of ligands. Our results are of practical importance for integrating TFCMNPs in chemiresistive sensing platforms and for (bio) and chemical sensing applications.

1. INTRODUCTION The use of thin film-capped metallic nanoparticles (TFCMNPs) in chemiresistors enables the realization of volatile gas sensors of low concentration detection levels.1−3 TFCMNPs are ideal for use in sensor arrays because they are chemically versatile, easily fabricated, low in cost, and readily integrated into chemiresistive sensing platforms using microelectrodes.4 An integration of TFCMNPs in chemiresistors requires controllability over the total properties of the thin film as well as regulated synthesis of the capped nanometal. Currently, nanometal synthesis shows superior controllability over chemical composition, size, shape, and variety of molecular ligands.5−8 However, fabricating thin films composed of NPs for sensing applications raises several issues that need to be addressed, such as roughness, role of capping ligands, stability over time and temperature, distance between the nanoparticles, packing density, and conductivity. Here, we fabricate the TFCMNPs by two self-assembly methods: layer by layer (LbL) and drop by drop, i.e., drop-casting (DC).9 Both methods are based on induced swelling and/or permittivity change of the medium between the NPs when exposed to an analyte.10 However, they differ conceptually: LbL is based on exchange reaction between the amine group (i.e., dodecylamine) and dithiols (i.e., 1,9 nonanedithiol or C9), and therefore, crosslinked network between the NPs is formed.9,11 DC is a direct © 2014 American Chemical Society

deposition technique from solution to substrate in which there are no chemical interbonds between the NPs. 12 The discrepancies between the two methods lead to TFCMNPs with different properties and diverse sensing behavior. Hence, by tailoring the parameters of each method, one can obtain the desired sensitivity and selectivity for a particular sensing application. In this work, we explore the two methods-based capped gold nanoparticles, stabilized with two different organic ligands (thiol and amine) for sensing polar and nonpolar analytes in the gas phase toward practical applications for medical diagnostics (e.g., lung, breast and liver cancer, chronic renal failure, etc.).1,3,6,13 TFCMNP devices prepared by LbL show a negative response (25%) to representative polar analytes, combined with low positive sensitivity to nonpolar analytes (3%). Additionally, the sensing mechanism is explained in terms of analyte-induced swelling of the NP film (in case of nonpolar analyte) and changes in the permittivity of the medium between the NPs (in case of polar analyte). However, in the DC method, the TFCMNP devices show significant negative response (600−800%) to representative polar and Received: August 21, 2013 Revised: January 7, 2014 Published: January 7, 2014 1903

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vast of organic materials and should be handled with extreme care); subsequently, the substrates were removed and placed into water and sonicated for 5 min and dried by N2. For investigating the electronic properties, the glass substrates were equipped with interdigitated gold electrode structures (finger pairs, 200 μm width and separation, and 105 nm height, including 5 nm titanium as an adhesion layer, and 100 nm of Au). Afterward, the slides were immersed in fresh 200 μL of 3aminopropyltriethoxysilane in 19.8 mL of toluene and heated to 60 °C for 30 min. Finally, the substrates were washed with toluene and water and dried by N2. The bare substrates showed an infinite resistivity. For LbL process, the substrates were treated as follows: the substrates were immersed for 15 min in solution of AuNPs in toluene with different concentration. (The low and high concentration of the particle solution corresponded to an absorbance of 0.28 ± 0.01 and 1 ± 0.01 which was measured at the maximum plasmon absorption band at 504 ± 2 nm). Thereafter, the substrates were washed with toluene for 1−2 s and treated with the linker solution for 15 min and washed again with toluene for 1−2 s. The linker solution contained 100 μL of alkanedithiol in 19.9 mL of toluene. The treatments with AuNPs, washing, linker solutions, and washing (to which we refer as one “deposition cycle”) were repeated up to 12−40 times. After each deposition cycle, the substrates were dried by nitrogen flow. The deposition of the AuNPs was monitored by measuring the conductance of the films, UV/vis spectra, and thickness after each deposition cycle. 2.4. Preparation of TFCMNPs by Drop Casting. Ten microliters of AuNPs was drop casted at the center of the device. The device dried for 1 h in a fume hood at ambient temperature and then baked at 50 °C in an oven for 4 h under low pressure (∼10−3 torr). 2.5. Apparatus. 2.5.1. Absorption. Absorption spectra of the solutions were taken with a UV/vis/near-IR (NIR) spectrophotometer (Model UV-1650, Shimadzu, Japan) in the 300−1000 nm wavelength region. 2.5.2. Scanning and Transmission Electron Microscopy. The morphologies of the samples were checked by SEM with a Zeiss HRSEM unique GEMINI-field emission column, and for crystallite and precise size we used TEM with a microscope (Model CM120, Philips, Netherlands) operated at 120 kV. For TEM measurements, samples were prepared by drop casting the diluted NP solutions (5 μL) onto 200-mesh carbon-coated copper grids. 2.5.3. Sensing. The developed sensors were mounted on a custom PTFE circuit board with 10 separated sensor sites that are mounted on a stainless steel test chamber with a volume of less than 100 cm3. An Agilent Multifunction switch 34980 controlled by USB was used to choose the active sensor at a given time. A Stanford Research System SR830 DSP lock-in amplifier controlled by an IEEE 488 system was used to supply the ac voltage signal and measure the corresponding current or resistance of the NP coating in the IME. The sensing responses presented herein were obtained at a fixed average voltage (200 mV, 500 Hz). The entire system was controlled by a custom Labview program. Using compressed-air source for drying, purified and oil free air was obtained. This gas was used as a carrier for the volatile organic compounds (VOCs). The vapors of different analytes were produced by bubbling carrier gas through them in their liquid states. The analyte-saturated airflow was diluted by air at a flow rate of 0.5 L min−1 .The sensing responses to single analytes were tested by introducing the analyte vapors to the test chamber using a constant air flow.

nonpolar analytes. The sensing mechanism is explained in terms of analyte-induced changes in medium permittivity between the NPs. At the same time, the DC devices exhibit high current drifts over time, due to incomplete release of absorbed chemical species. On the other hand, LbL devices show low drift with a precise control of absorbance and conductivity during the deposition process. The sensing capabilities for both methods imply that each method can be used for different applications. For example, DC is suited for gas sensing after controlling the drift limitation while LbL can be useful for pressure sensing.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O, 99%), tetraoctylammonium bromide (TOAB, 99%), sodium borohydride (NaBH4,99%), 2-nitro-4-trifluoromethylbenzenethiol (NTFMBT), 2-amino-4-chlorobenzenethiol (ACBT), dodecylamine (DA), and dodecylthiol (DT) were purchased from Sigma-Aldrich. Analyte vapors were generated from hexane, ethanol, and ethyl benzene, also purchased from Sigma−Aldrich. The water vapor was obtained from 18 MΩ cm resistivity deionized (DI) water which was purified with a Millipore Nanopure water system. 2.2. Synthesis of Bicapped Ligands AuNPs. HAuCl4· 3H2 , 2.53 × 10−4 mol (0.1 g), was dissolved in 8 mL of water for 1 min. The aqueous phase was executed using 8.96 × 10−4 mol (0.49 g) of TOABr in toluene (27 mL) for 10 min with vigorous stirring. Then, the phases were separated (the aqueous phase should be completely clear), and the organic phase was kept under ice temperature (4 °C). One half of the Au(III) solution, [AuCl4−+N(C8H17)4], was added to 0.043 g of DA in 4 mL of toluene. The solution complex [C12H25 NH2· AuCl4−+N(C8H17)4] was kept under ice temperature and stirred for 30 min. The color was shifted from red to deep red− black after the immediate addition of DA and changed in time to light yellow. For the second half of Au(III) solution, we added 0.25 × 10−4 mol thiol solution (i.e., 0.0058 g of NTFMBT), which was dissolved in 4 mL of toluene. The solution was kept under ice temperature and stirred for 30 min. The two halves were mixed together and stirred for 10 min. NaBH4, 3.3 × 10−3 mol (0.127 g), was dissolved in 8 mL water and cooled down to 0 °C. Thereafter, we rapidly added all the 8 mL in 2−3 s to the cooled gold solution. To get monodisperse nanoparticles with the two ligands, the reaction was allowed to occur under various stirring (1200 rpm) at ice temperature for 3 h, statistically producing a red-wine color solution of biligands-capped Au nanoparticles (AuNPs). In case we need only amine groups as a ligand, we added 0.048 g of DA (instead of 0.043 g) to the whole Au solution. The phases were separated, and the solvent was removed by slow rotary evaporation at room temperature and followed by multiple washing using 50 mL of cold ethanol (10 °C). After the first wash, the solution was kept at 5 °C for 18 h until complete immersing. Finally, the solvent was evaporated, and the NPs were immersed in 20 mL of toluene and kept at 10 °C for further use. 2.3. Preparation of TFCMNPs by Layer by Layer. Prior to use, the substrates (silicon wafers and glass slides) were sonicated with acetone and 2-propanol for 1 min and dried by N2. The treated substrates were exposed to UV. Then, the substrates were immersed for 10 min in 1:1:1 (v/v) mixture of concentrated chloride acid (37%), water, and 30% hydrogen peroxide (caution: this piranha solution reacts violently with a 1904

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Figure 1. Normalized plasmon spectra of AuNPs as proceeded along the reaction time: (a) biligand-capped AuNPs, (b) monoligand-capped AuNPs.

Figure 2. (a) UV/vis of biligand-capped AuNP solution after first, thirrd, and sixth month. (b) UV/vis of monoligand-capped AuNP solution after first, third, and sixth month. (c) UV/vis of the bicapped AuNP solution annelid at 75 C° along the annealing time. (d) UV/vis of the monoligandcapped AuNP solution annelid at 75 C° along the annealing time.

Signals were collected for 10 min under dry air flow, followed by 10 min under flowing analyte vapor, and then followed by another 10 min of dry air flow. The cycles were typically repeated three times to test reproducibility.

sensing properties of the TFCMNPs as will be shown later. It is worth mentioning that the DA molecule has an additional function that binds the AuNPs together during the LbL process through an exchange reaction between the amine and the dithiol groups.4,9,11 To follow the biligand-capped AuNP syntheses, the absorbance of the AuNPs was characterized by UV/vis spectroscopy as a function of reaction time as shown in Figure 1. Figure 1a shows the normalized UV/vis spectrum (nucleation and growth) of the AuNP nucleation and growth. In the nucleation stage (0−10 s), the temporary complex (exhibited a pink-red color) of the amine ligand [C12H25 NH2· AuCl4−+N(C8H17)4] and thiol ligand [AuCl4−+N(C8H17)4] decomposed and formed Au seeds and showed a low absorption peak at 488 ± 10 nm. In the growth stage 10s− 180 min, two steps were observed: (a) step I, rapid growth of AuNPs (10 s−1 min): the solution color changed to dark red, and high peak absorption was rapidly red-shifted to 500 ± 5 nm indicating the growth of the AuNPs;20 (b) step II, slow growth of AuNPs (1−180 min): the absorption spectra exhibited sharper plasmon-resonance bands along the reaction time and were slow red-shifted from 500 ± 5 nm to 510 ± 10 nm (see

3. RESULTS AND DISCUSSION 3.1. Synthesis of Biligands-Capped AuNPs. The formation of stable AuNPs capped with monoligands was first established in 1994 by Brust et.al.14 On the basis of his finding, several results of these particles were published.15 In our work, the growth technique holds a fundamental change. The growth step occurs during the stabilizing step after the addition of two ligands simultaneously (NTFMBT and DA) forming biligandcapped gold seeds. Biligand-capped AuNPs continuously grow after NP nucleation. By applying a few modifications to the original synthesis (e.g., concentration, molar ratio between the two ligands), we were able to observe the growth of biligandcapped AuNPs which shows good promise in sensing applications.16−19 TEM characterization of the obtained AuNPs shows a narrow size distribution with 5 nm in diameter (see Supporting Information, Figure_S1). The two ligands were essential to determine the stability of the AuNPs and the 1905

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Figure 3. (a) Conductivity of the TFCMNP devices as function of the number of deposition cycles in LbL and DC methods. (b) Normalized average conductance at t = 0 of the TFCMNPs versus exposure time to air, as determined by the NR = Rt/Rt=0 measurements. Full scale of the inset SEM images is 500 nm.

by 30 ± 4 nm while bicapped ligand AuNP solution was more stable and showed a comparable red-shift (37 ± 2 nm) only after annealing to 60 min (cf. Figure 2c and d). Increasing the annealing time from 60 to 90 min shifts furthermore the absorption spectra from 520 ± 10 nm up to 570 ± 10 nm for both AuNP solutions. The shifts along the temperature indicates further facile size growth and high size or/and shape distribution of the NPs as can be shown in Figure S3b of the Supporting Information and as illustrated from the UV/vis spectra of Figure 2c and 2d. The figure shows a TEM image of the annealed biligand-capped AuNPs with different sizes and shapes: more than 50% of the nanoparticles were 10−15 nm with spherical shape, 20% were transformed into rodlike shapes with a size of 20−50 nm, 20% were transformed into triangle-like shapes with a size of 20−25 nm, and the rest of the nanoparticles ( 0, then γ(20−80) ≈ 0, while for the DC, the γ was decreasing along the time γ(0−3) ≪ 0, γ(3−20) < 0, then γ(20−80) ≤ 0 This illustrates that TFCMNP devices are more stable in LbL method than DC. The γ explained by the remaining solvent (toluene) in the TFCMNPs by using DC: a lot of solvent is casted on the surface, adheres between the NPs, and evaporates slowly. When the solvent evaporates, the NPs are getting closer, and the conductivity increases in time. 3.3.2. Capped Ligands. It is found out that the conductivity of TFCMNP devices can be tuned by the capped ligands giving control over these devices. The structure and the number of methylene units −(CH2)n− of the ligands play a major role in determining the conductance of the TFCMNPs. Figure 4 shows the conductivity of the TFCMNPs with three different ligands: DT with n = 12 methylene units, TFMBT and ACBT with n = 6 methylene units. The NTFMBT and ACBT have a benzene ring but differ by their functional groups. NTFMBT has F3C and NO2 while ACBT has Cl and NH2, which makes the NTFMBT more polar than ACBT. The TFCMNPs with the mentioned ligands were prepared by LbL as described in the Experimental Section. As shown, the longer the methylene units, the lower is the conductivity, which explains the low conductivity of AuNPs capped by the DT ligands. Interestingly, the NTFMBT shows higher conductivity than ACBT films which might spout from the polar functional groups which increase the particle−particle interaction through dipole− dipole interaction. It should be noted that DC method was not affected by the type of the capped ligands. 3.3.3. Vapor-Sensing Properties of TFCMNP Devices. We compared the TFCMNP properties of the two methods with the same AuNP solution (here we used NTFMBT−AuNP). NTFMBT has a hydrophobic aromatic structure “toluene-like” that is expected to enhance the sensitivity toward aromatic analytes. Additionally, it has a hydrophobic group (CH3) and

explained shortly later. In the second stage, a significant increase of conductance was observed, signifying that the layers started to be formed and a thin full film is already established. It should be noted that an almost linear increase in conductance was obtained indicating that each deposition cycle contributes the same amount of nanoparticles to the deposited layer. In the third stage, the resistivity decreased slowly below 2 ± 1 MΩ to several hundreds of KΩ since continuous network and layers had already formed (see also Figure 4).4,9,11 However, in the

Figure 4. Conductivity of TFCMNP devices with different ligands prepared by LbL. The conductivity of the TFCMNP devices was dependent on the ligand type.

DC method, the TFCMNPs immediately show low resistivity of several MΩ to KΩ after two drops only. Adding more drops may decrease or increase the conductivity of TFCMNPs with unclear and uncontrolled trend. This tendency was similar to the absorbance properties of the TFCMNPs: the LbL shows gradual increase of absorbance along the deposition cycles, while DC shows high absorbance after two drops (cf. Figure S5 of the Supporting Information). Compared to the solutionphase absorbance spectrum, (λmax = 505 nm), the absorption is broadened and significantly red-shifted (λmax = 550 nm) which is probably due to differences in the dielectric environment of the nanoparticles as well as particle−particle interactions.31,32 The morphology of the TFCMNPs was affected by the used method. Representative SEM images of the two TFCMNP devices are shown in the inset of Figure 3a. LbL displays wellordered and smooth TFCMNPs. For example, the low bottom inset SEM image shows two smooth subsequent layers; top layer and down layer. A high-resolution SEM image of the top layer shows AuNPs with diameter distribution inside the layer between 5 and 8 nm (cf. Figure S6 of the Supporting Information), which is in good agreement with the diameter of the AuNPs used for film preparation. However, DC forms

Figure 5. Normalized resistance, ΔR/Rb, of the TFCMNPs prepared by LbL and DC methods. Upon exposure, hexane, ethylbenzene, water, and ethanol in the vapor phase at pa/po = 0.5. (a) shows both responses in the same scale bar, while (b) shows the response of LbL device only. 1907

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The positive substantial response observed in LbL can be attributed mainly to film swelling, which leads to an increase of the interparticle distances. The swelling along the analyte exposure was observed and measured by the change in the thickness of TFCMNPs and measured with ellipsometry as can be shown in Figure_S7 of the Supporting Information. For example, the positive response in hexane show a 0.6% increase in the thickness. Even though water shows 2% increase in thickness, it shows negative response. The negative response for the polar analytes is explained by the high dielectric constants (of water and ethanol than NTFMBT) that increases the medium’s permittivity and, hence, decreases the resistivity in average. This effect is supported by the TFCMNP sensing behavior prepared by DC method which has excess water as discussed above.1,3,6,33

hydrophilic (F3C and NO2) group. Therefore, these ligands are expected to enhance the affinity to both polar and nonpolar analytes. A supply of purified, dry air was split into two streams, one of which was used as carrier gas. The other one was directed through a bubbler containing the liquid analyte, to generate semisaturated analyte vapor with pa/po = 0.5 and is directed to the exposure chamber for a time interval of 10 min, followed by 10 min of carrier gas flow. The exposure chamber was continuously flushed with the carrier gas. The set of analytes used for this study included a nonpolar alkyl analyte (hexane), a polar aromatic analyte (ethyl benzene), and polar analytes (ethanol and water; cf. Figure 5). Figure 5 shows the time dependence of the response, ΔR/Rb = (R − Rb)/Rb, of the TFCMNPs to pulses of the above analytes. R is the steady state resistance of the sensor when it is exposed to the analyte, and Rb is its baseline resistance when flushed with dry air, in the absence of the analyte. Rb was typically ∼2MΩ for TFCMNP devices. Note that it is customary to normalize the resistance change, R − Rb, with respect to Rb, because it allows the comparison of sensors with different baseline resistances. In LbL method, the response of the TFCMNPs (i.e., 90% of the final response state was reached) was fast (below 5 s), while in the DC, the response took 3−5 min. The responses of the TFCMNP devices are fully reversible upon switching off the flow of analyte vapor into the chamber and purging with reference gas. The low response values in the LbL method (3− 25%) relative to DC (600−800%) are a direct result of the crossed NP nature of the TFMCNPs in LbL which limits the swelling. As illustrated by SEM images (top inset image in Figure 3a and Figure S6 of the Supporting Information), the TFCMNP is symmetrical in the three axes and has almost the same density in all directions (like a crystal). Therefore, only the Z-component may contribute to the response. However, the TFCMNP prepared by the DC has only local order; thus, all the components (x, y, z) may contribute to the response. Another remarkable difference to be addressed is the drift in Rb(t) which was obtained in DC when measuring the TFCMNPs over long periods of time (20 MΩ/h). However, the Rb(t) of the LbL shows a minor and linear drift along the time, and overall there is no significant change (10 KΩ/h). This is most probable due to one or more combinations of the following reasons: (i) a gradual depletion of water molecules and contaminants present in ambient air to which the sensors were exposed before they were introduced into the exposure chamber or (ii) a gradual accumulation of analyte molecules during cyclical exposure (iii) not fully dried up layers where the solvent may continue to evaporate during analyte exposure.12 However, the differences in sensitivity and response directions were governed by the polarity of analyte molecules and the ligands capping the AuNPs. Two important deductions should be considered: (1) Negative response for all analytes when DC was used while LbL shows negative response only to H2O and ethanol and positive response to hexane and ethylbenzene, (2) TFCMNP prepared by DC shows the highest response values up to (ΔR/Rb) = 600−700% upon exposure to nonpolar alkyl analytes (hexane) and aromatics (ethyl benzene) and lower response to the polar analytes, especially to water, which was almost an order of magnitude lower (80%). This is expected because of the hydrophobic structure of “toluene structure” of the capped ligand.1 The sensing mechanism can be explained in terms of two competing processes: analyte-induced swelling of the NP films and changes in the permittivity of the medium between the NPs.

4. SUMMARY In summary, we have adopted a seed-mediated growth method with biligands to synthesize AuNPs and to integrate them into TFCMNP devices. The biligand-capped AuNPs were spherical with 5 nm in diameter. The structural, morphological analysis, and stability measurements indicate that AuNPs with thiol ligands are more stable than AuNPs with amine ligands. After annealing, the biligand-capped AuNPs were transformed to quasi-spherical shapes like rods, hexagons, and triangles. To use the new properties of biligands for sensing application, two methods were developed: (a) LbL and (b) DC. In this study, we compared the properties of the sensing devises based on the two methods. Deposition of the NPs has a controlled manner in LbL method than the DC method; thus, ordered thin films are formed. However, DC method produced TFCMNP devices with significantly higher electrical responses upon exposure to various analytes than similar TFCMNP devices prepared via LbL method. These differences are explained by the higher swellability and medium permittivity observed in the TFCMNPs prepared by DC than LbL. The LbL devices show low response which is applicative for high-pressure sensors where stable and reproducible sensors are required.

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ASSOCIATED CONTENT

* Supporting Information S

Figures S1−S7 as described on the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: + 49 9131 6877 552. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.Y.B gratefully acknowledges the Max-Planck Society for the Post-Doctoral fellowship. The authors acknowledge the financial support by the FP7 EU project LCAOS (no. 258868, HEALTH priority). The authors appreciate the support of Max Planck Institute for the Science of Light− Erlangen, Germany.

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dx.doi.org/10.1021/jp4083823 | J. Phys. Chem. C 2014, 118, 1903−1909