Thermal Annealing: A Facile Way of Conferring Responsivity to Inert

Oct 14, 2014 - All of these results indicated that thermal annealing was an efficient way to confer responsivity to inert Au NP arrays. Together with ...
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Thermal Annealing: A Facile Way of Conferring Responsivity to Inert Alkyl-Chain-Passivated Nanoparticle Arrays Jun Zhou,†,‡ Guoshuai Song,† Yan Li,† Youxin Song,§ Bin Chen,§ Xuemin Zhang,† Tieqiang Wang,*,† Yu Fu,*,† and Fei Li‡ †

College of Sciences, Northeastern University, Shenyang, Liaoning 110819, People’s Republic of China State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun, Jilin 130012, People’s Republic of China § Chengde Medical College, Chengde, Hebei 067000, People’s Republic of China ‡

ABSTRACT: This work demonstrates a facile post-treatment strategy, vacuum thermal annealing, to fabricate a dodecanethiol-passivated gold nanoparticle (Au NP) array with organic solvent sensitivity. Through investigating the structure change of the Au NP array, it was found that the interparticle distance decreased during vacuum heat treatment, which meant a closer arrangement of the particles and a more dense packing of the dodecanethiol ligands in the interparticle region. The condensation would increase the interaction of the alkyl chain and enhance their interdigitation. Furthermore, on the basis of the stretching of the alkyl chains in organic solvents, the thermally treated Au NP array showed a good response to organic solvent or vapor by using the interdigitated dodecanethiol network as its responsive unit. The alkyl chains stretch to different extents in different organic solvents, leading to differences in interparticle distance, which provided a distinct blue shift of maximum wavelength upon exposure to various organic solvents or vapors. All of these results indicated that thermal annealing was an efficient way to confer responsivity to inert Au NP arrays. Together with the cost-effectiveness of such NP arrays, this study has potential in the development of economical sensors for medical diagnostics, food safety screening, and environmental pollution monitoring.

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INTRODUCTION Plasmonic nanostructures are a topic of current interest because of their ability to confine and manipulate light at the nanoscale as well as their potential for broad application as, inter alia, surface-enhanced spectroscopic materials, chemical or biosensors, and metamaterials.1−6 A prominent feature of plasmonic nanostructures is that their optical properties can be systematically tuned by varying their geometry parameters.7−15 To that end, it is vitally important to develop efficient, reliable methods of fabricating functional plasmonic nanostructures with precise geometry control. At present, plasmonic microstructures are mainly fabricated by two strategies: topdown lithography16−21 and bottom-up self-assembly.22−26 Although top-down lithography techniques have shown an ability to control the nanostructures at the nanometer scale, most of these techniques require expensive equipment and suffer from the disadvantage of being time-consuming, which greatly restricts the application of plasmonic microstructures. In comparison to top-down lithography, bottom-up self-assembly of noble-metal nanoparticles is an attractive strategy to obtain ordered, uniform nanostructures, because of its cost and timeefficiency. Up to now, many noble-metal nanoparticle arrays have been successfully fabricated through self-assembly methods for various applications, involving enhanced spectroscopy and sensors.26 Therefore, considering the future applications of plasmonic nanostructures, research into their © XXXX American Chemical Society

fabrication and unique functions based on cost-effective selfassembly strategies and the use of their unique functions remains ongoing. For nanoparticle array self-assembly, the organic ligand capped on the nanoparticle plays a crucial role.27−29 Besides stabilization of the nanoparticles and the prevention of their aggregation in the solution, it also provides the necessary repulsive force and spatial coherence to balance the internanoparticle attractive force during the assembly process. Because of the roles played by the ligands in the nanoparticle solution and self-assembly process, the types of molecules that could be used as ligands in the assembly of such an array were limited. Therefore, to confer unique functions to the nanoparticle arrays, it is usually required to introduce special organic ligands, such as DNA or molecular agents with multifunctional groups, by post-ligand exchange with the self-assembled nanoparticle arrays.30−35 Obviously, such strategies are both complicated and tedious; therefore, the treatment conditions must be strictly controlled, which limits the development of nanoparticle arrays for further application. Until now, one of the most widely used ligands was alkyl sulfhydrate, which is commonly considered as an inert molecule.36,37 Because the Received: August 28, 2014 Revised: October 6, 2014

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RESULTS AND DISCUSSION Fabrication and Thermal Annealing Treatment of Au NP Arrays. First, Au NP arrays were fabricated through an interface assembly method as previously reported.36 Next, the self-assembled Au NP array was heated at 80 °C in vacuum for 3 h and cooled to room temperature. After vacuum thermal annealing, the color of the Au NP array changed from purplish red to purplish blue, as shown in the inset to Figure 1. Through

synthesis and assembly of alkyl-sulfhydrate-stabilized nanoparticles is well-established, it would be of interest to confer unique functions to the nanoparticle arrays stabilized with alkyl sulfhydrate through facile treatment. In this study, a facile post-treatment, vacuum thermal annealing, was performed to confer solvent responsivity to an inert alkyl-chain-passivated gold nanoparticle (Au NP) array. Through investigation of the structural changes during thermal annealing, it was found that the interpatricle spacing of the array was decreased, which would generate more efficient ligand packing and enhance the interdigitation of the alkyl chains coating the Au NPs. Also, owing to the interdigitation of the alkyl chains of the dodecanethiol ligand, which could stretch to different extents in various organic solvents, the thermally treated Au NP array was sensitive to external stimulus, such as the presence of an organic solvent. The different stretching extents of the alkyl chains would lead to differences in the interparticle spacing, providing a distinct blue shift of the maximum wavelength upon exposure to various organic solvents or vapors (ethyl acetate, acetone, acetonitrile, ethanol, and methanol). Additionally, the as-prepared Au NP array also had a rapid response rate and excellent reproducibility. Therefore, together with the cost-effectiveness of the Au NP array, it was believed that this Au NP array would be an excellent sensing platform for organic solvent detection in many applications.

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Figure 1. Absorption spectra of the Au NP array before and after heat treatment. The insets show the Au NP array samples (ϕ = 5 mm) before and after thermal annealing.

EXPERIMENTAL SECTION measurement of the absorption spectrum of the Au NP array, it was found that the maximum absorption wavelength (λmax) redshifted from 558 ± 2 to 580 ± 3 nm (Figure 1). According to the literature,39 λmax of the Au NP array was related to its interparticle spacing and λmax shift toward red or blue indicated the decrease or increase of the interparticle spacing. Therefore, by comparing the absorption spectra before and after thermal annealing, it was inferred that the interparticle spacing of the Au NP array decreased after thermal annealing. To prove the speculated spacing change, TEM and SAXS measurements were performed to investigate the structural change during annealing (Figure 2). The TEM images of the Au NP array before and after heat treatment are shown in panels A and B of Figure 2; when these two TEM images were compared, it was clear that the Au NPs became much denser and the period of the array decreased after heat treatment. Meanwhile, the SAXS result (Figure 2C) showed an increase in the grazing angle from 1.21 to 1.26, which also indicated a decreased lattice constant of the Au NP array according to the Bragg diffraction formula. Because the size of the Au NPs did not change, as shown in the TEM images, it could be concluded in both TEM and SAXS measurements that the interparticle spacing decreased during annealing; this corresponded well with the conclusion drawn from consideration of the absorption spectra of the Au NP arrays. According to the literature,40 the length of a DDT ligand is about 2.3 nm (l = 0.83 + 0.122n, where l represents the length and n is the number of carbon atoms per chain); thus, if the alkyl chains of a DDT molecule were stretched to their fullest extent and the Au NP array was loosely packed, it could be calculated that the distance between neighboring nanoparticles was ca. 4.6 nm. However, both TEM and SAXS measurements have demonstrated that the interparticle spacing was only about 1.6 nm, which was much smaller than that calculated; this therefore indicated that the alkyl chains of the DDT ligand

Materials. Quartz substrates were cleaned by immersion in piranha solution (3:1 concentrated H2SO4/30% H2O2) for 1 h at 70 °C to create a hydrophilic surface and then rinsed repeatedly with Milli-Q water (18.2 MΩ cm) and ethanol. Teflon tape was purchased from SPI Supplies (West Chester, PA). All of the substrates were dried in nitrogen gas before use. Chloroauric acid (HAuCl4·4H2O), 1dodecanethiol (DDT). and sodium borohydride (NaBH4) were purchased from Alfa Aesar and used without any further purification. All of the solvents (hexane, toluene, acetonitrile, ethyl acetate, acetone, methanol, and ethanol) were used as received. Preparation. The close-packed Au NP monolayer was fabricated by evaporation-induced assembly according to the literature.36 Briefly, dodecanethiol-coated Au nanoparticles (5.7 ± 0.5 nm) were first prepared by reduction of HAuCl4 with NaBH4 in water and transferred into a hexane solution of dodecanethiol. Next, a droplet of the Au NP solution was deposited on a larger toluene droplet on a Teflon tape. After the hexane−toluene droplet evaporated away, a close-packed and ordered Au NP monolayer was obtained. Finally, to confer solvent responsibility to the Au NP array, the Au NP arrays were vacuumthermal-treated at 80 °C for 3 h and then cooling in atmosphere. Characterization. To facilitate the characterization of the Au NP arrays, the Au NP array was transferred from Teflon tape to quartz substrate through a transfer technique reported before.38 The transmission electron microscopy (TEM) images of the Au NP arrays were obtained through a Hitachi H-7650 electron microscope. Smallangle X-ray scatting (SAXS) was measured on a Rigaku SmartLab. Ultraviolet−visible (UV−vis) spectra of the Au NP arrays were measured using Persee TU-1810 spectroscopy. The time-resolved spectra were measured using a custom-made setup consisting of a collimated beam of a fiber-coupled tungsten−bromine lamp (Ocean Optics) coupled to a spectrometer (Ocean Optics, Maya 2000PRO). To measure the spectra of the sample with organic solvent, a cuvette was first filled with different organic solvents and then the sample was placed in the cuvette for measurement. When measuring the spectra of the sample with organic vapor, the sample was first placed in an empty cuvette, then several drops of volatile organic solution were dropped into the cuvette, and the cuvette was sealed to obtain the saturated organic vapor atmosphere. B

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spacing is closely related to the plasmonic optical properties of the Au NP arrays, it was possible to distinguish organic solvents by monitoring the plasmonic optical properties of the Au NP arrays in different organic solvents. In most cases, the λmax value of plasmonic nanostructures was also sensitive to the surrounding medium [refractive index (RI)];42,43 thus, to exclude the influence of the RI of the surrounding medium, organic solutions with similar RI were used to investigate the solvent responsivity of Au NP arrays. The absorption spectra of Au NP arrays immersed in different organic solvents were measured, and the maximum absorption wavelength λmax of the absorption spectra are listed in Table 1, where it was seen that Table 1. λmax for a Representative Au NP Array Sample in Different Organic Solvents solvent

λmax (nm)

RI

polarity

H2O methanol acetonitrile ethanol acetone ethyl acetate

572 572 570 567 564 557

1.33 1.33 1.34 1.36 1.36 1.37

10.2 6.6 6.2 6.0 5.4 4.3

λmax blue-shifted from 572 ± 3 to 557 ± 2 nm as the polarity of the solvent decreased from 10.2 (H2O) to 4.3 (ethyl acetate). Because the RI of the surrounding medium did not change, it was believed that the different extensions extent of the DDT ligand in different organic solutions resulted in the blue shift of λmax. Additionally, because the color of the nanoparticle array sample changed in only several seconds when the sample was placed into the cuvette filled with solvents, it is believed that the response time of the sample against solvent is about several seconds. As was known, for a mixture of two solvents with different polarities, the polarity of the mixture could also be changed according to the mixing proportions; thus, the concentration of one solvent in a two-component mixture could also be measured according to the λmax value of the Au NP array. Figure 3 shows measured λmax values in a two-component mixture (methanol and ethyl acetate with different proportions thereof). As shown in Figure 3, the λmax value of the Au NP array blue-shifted monotonically from 572 to 557 nm as the concentration of ethyl acetate (the lower polarity solvent) increased. This indicated that the interparticle spacing of the Au

Figure 2. TEM images of Au NP arrays (A) before and (B) after heating in vacuum. (C) SAXS spectra of the Au NP arrays (red) before and (black) after heat treatment.

might have collapsed and partly interdigitated with each other before thermal treatment. Additionally, after vacuum heating, the interparticle spacing had decreased further to ca. 1.2 nm, meaning that the packing status of the ligand had become much denser and the interdigitation of alkyl chains was further enhanced during thermal annealing. Finally, a more efficient packing of the Au NPs was generated; this result was similar to the work reported by Wang and co-workers.41 In addition, after vacuum thermal treatment, the stability of the thermally treated Au NP arrays against an organic solvent was enhanced; they could survive in many organic solvents, including ethyl acetate, acetone, acetonitrile, ethanol, and methanol. It was believed that the interdigitation and efficient packing of the alkyl chains was the reason for the improvement in stability thereof. Response of the Au NP Array to Organic Solvent and Vapor. Except for the stability of Au NP arrays in organic solvents, vacuum thermal treatment would also confer another unique function, solvent responsivity, on the Au NP arrays based on the further interdigitation of the DDT ligands. Owing to the interdigitation of the alkyl chains of the DDT ligand, the as-prepared NP array could be regarded as a physically crosslinked network and the interdigitation of the alkyl chains serves as the cross-linked point of the whole nanoparticle array. Thus, similar to the swelling properties of cross-linked polymers, the interdigitated chains would stretch to different extents in organic solvents with different polarities and the interparticle spacing would also change accordingly. Because the mainly responsive unit of the array is the DDT ligand, if the NP arrays was placed in low-polarity solvent (good solvent of the DDT molecular), the stretching extent of the DDT molecular will be greater, while if the solvent is high-polarity, the stretching of the DDT molecule will be suppressed; thus, the interparticle spacing will not increase accordingly. Because the interparticle

Figure 3. λmax against the concentration of ethyl acetate (ω) in the two-component mixture solution (the other solvent is methanol). C

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Figure 4. (A) Absorption spectra of Au NP arrays measured in different organic solvent vapors. (B) SAXS curves of Au NP arrays in (black) air, (red) ethyl acetate vapor, and (blue) chloroform vapor.

during one sensing cycle under different organic solvent vapors. Figure 5 shows a typical plot of the absorption λmax of the Au

NP arrays increased as the polarity of the mixture weakened; this agreed with the conclusion drawn from tests in pure solvent. Therefore, this Au NP array could be used as a sensing platform with selectivity for the specific polarity of an organic solvent. In particular, as shown in Figure 3, the λmax value of the Au NP arrays blue-shifted significantly when the percentage (ω) of ethyl acetate was greater than 80%. According to the literature,44 if the polarity of a solvent matched the polarity of the ligand, the extent of the stretching of the ligand chains would change dramatically. Therefore, it was inferred that this specific polarity of mixed solvent matched the polarity of the DDT ligand, resulting in an increase in the stretching of the ligand. This would generate a mutual repulsion between nanoparticles; thus, the interparticle spacing would also increase, and the value of λmax of the Au NP array would be blue-shifted by this interparticle repulsion. Except for its solvent sensitivity, the as-prepared Au NP array was also responsive to organic solution vapors. The λmax variation of the Au NP array in different organic solvent vapors was monitored as shown in Figure 4A. It was found that the λmax value of the Au NP array blue-shifted from 576 to 560 nm as the solvent vapor changed from high-polarity methanol vapor to low-polarity chloroform vapor. Interestingly, such shifting followed the same trend as that for the solvent sensing behavior of the Au NP array; it was therefore believed that the vapor sensitivity was also attributable to the change in interparticle spacing caused by the stretching of the DDT ligand. To verify the DDT stretching-based response mechanism, SAXS measurement of the Au NP array exposed to various organic vapors was performed, as shown in Figure 4B. As the vapor changed from air to chloroform vapor, the diffraction peak value of the array gradually decreased, directly proving the change of interparticle spacing in the presence of different types of vapor. When the absorption and SAXS results were compared, it could be seen that the responsivity of the asprepared Au NP arrays was based on the stretching of the DDT ligand under different external polarity atmospheres, which is similar to the swelling of cross-linked polymer in the vapor atmosphere of its good solvent.45 For sensing applications, rapid detection is vital to the performance of a sensor and the response time is an important parameter in the evaluation of sensing performance for further application. Thus, the response time of the Au NP array was examined by measuring the time-resolved absorption spectrum

Figure 5. λmax for the Au NP array versus time during one sensing cycle in ethyl acetate vapor.

NP array versus time during one sensing cycle in the presence of ethyl acetate vapor. As shown in Figure 5, once the ethyl acetate vapor was introduced to the Au NP array, it took ca. 12 s for λmax to shift from 578 to 565 nm. The value of λmax of the Au NP array would remain at around 565 nm in saturated acetic ether vapor. After the cap of the sealed cuvette was removed and the acetic ether vapor was slowly volatilized, the λmax value of the Au NP array immediately red-shifted and returned to its original value of 578 nm in only 13 s, which was close to the response time following the introduction of ethyl acetate vapor. Except for the ethyl acetate vapor, the response time of the Au NP arrays to other organic solvent vapors was also measured, and the results are listed in Table 2. It could be seen that the thermally treated Au NP arrays all showed a fast response to different kinds of solvent vapor (the response times Table 2. Response Time of the Au NP Array against Different Organic Vapors vapor air acetone vapor ethyl acetate vapor chloroform vapor D

λmax (nm) 580 569 565 560

± ± ± ±

3 2 2 3

response time (s) 10 ± 1 12 ± 2 9±1

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were all of the order of several seconds). It was believed that the fast response of the Au NP arrays may be attributable to the open architecture of the array, which facilitated the accessibility of external analytes (vapor) to the sensitive unit (DDT ligand). Therefore, the as-prepared Au NP arrays could be used for quick detection of organic vapor. Besides the response time, reproducibility is another important parameter related to sensing performance. Some 20 cycles of the solvent sensing process were performed, as shown in Figure 6; the Au NP arrays exhibited good stability

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21174024 and 21404021), the Supporting Projects for Talents in the Universities of Liaoning Province (LR2012008), and the Fundamental Research Funds for the Central Universities (N120505005, N120405007, N130305002, and N130205001).

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Figure 6. Reproducibility of the as-prepared Au NP array during repeated immersion into acetic ether and ethanol.

and excellent reproducibility for practical sensing applications. Almost no obvious differences in the peaks in each of the λmax values of the Au NP arrays in acetic ether and ethanol were observed between each cycle. In this experiment, it was found that this responsive behavior to water vapor could be repeated over 50 cycles and the sensing performance was maintained for at least 1 month. Because the DDT ligand was deeply interdigitated, its structure was well-preserved during several sensing processes, which induced the excellent reproducibility of the Au NP arrays. Owing to the fast response rate and excellent reproducibility, the as-prepared Au NP array was an attractive candidate for future sensing applications.

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CONCLUSION

In summary, Au NP arrays with solvent responsivity were fabricated by a facile post-heating treatment of inert alkyl-chainpassivated Au NP arrays. On the basis of the absorption spectra, TEM, and SAXS measurements, it was found that the interparticle spacing decreased during the post-heating treatment, owing to further interdigitation of the DDT ligand. Owing to the stretching of the interdigitated DDT ligand in various surrounding conditions, the as-prepared Au NP arrays showed a good response against various organic solvents and vapors. Importantly, the as-prepared Au NP arrays had fast response rates and excellent reproducibility; this made them attractive candidates for future sensing applications. Therefore, together with their cost-effectiveness and stability, it was believed that such Au NP arrays would form an excellent sensing platform for organic solvent detection in medical diagnostics, food safety screening, and environmental pollution monitoring fields. E

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