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Thermoresponsive polymer-stabilized silver nanoparticles
Journal of Colloid and Interface Science 319 (2008) 175–181 www.elsevier.com/locate/jcis
Thermoresponsive polymer-stabilized silver nanoparticles Limin Guo a,1 , Jingjing Nie b , Binyang Du c,∗ , Zhangquan Peng a,∗,1 , Bernd Tesche d , Karl Kleinermanns a,∗ a Institut für Physikalische Chemie, Heinrich-Heine-Universität Düsseldorf, 40225 Düsseldorf, Germany b Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China c MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, PR China d Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
Received 10 September 2007; accepted 19 November 2007 Available online 24 November 2007
Abstract Silver nanoparticles (Ag NPs) stabilized by a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), have been synthesized by the reduction of silver ions with NaBH4 in aqueous solutions. The obtained Ag NPs are very stable at room temperature due to the extended coil conformation of the PNIPAM chain at temperatures below its volume phase transition temperature (∼32 ◦ C). At higher temperatures (such as 45 ◦ C) above the phase transition of PNIPAM, only minute aggregation between Ag NPs was observed, showing that the collapsed PNIPAM chains still retain the ability to stabilize Ag NPs. The PNIPAM-stabilized Ag NPs were then characterized as a function of the thermal phase transition of PNIPAM by UV–vis spectroscopy, dynamic light scattering, transmission electron microscopy, and cyclic voltammeter. Consistent results were obtained showing that the phase transition of PNIPAM has some effect on the optical properties of Ag NPs. Switchable electrochemical response of the PNIPAM-stabilized Ag NPs triggered by temperature change was observed. © 2007 Elsevier Inc. All rights reserved. Keywords: Silver nanoparticles; Thermoresponsive polymer; Optical property; Electrochemistry
1. Introduction Surface modification of noble metal nanoparticles (NPs) with stimuli responsive polymers, their optical properties and the assemblies of such modified NPs are subjects of extensive research efforts [1–8]. Noble metal NPs (such as Au and Ag) often exhibit strong plasmon resonance absorption in the visible spectrum, which depends on the size, shape and surface chemistry of the NPs [9–12]. Furthermore, while the composition of the NPs may be held constant, its plasmon resonance absorption maximum could be shifted hundreds of nanometers by changing its shape, orientation in the incident light, or the * Corresponding authors. Fax: +86 571 87952400.
E-mail addresses: [email protected] (B. Du), [email protected] (Z. Peng), [email protected] (K. Kleinermanns). 1 Present address: Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark. 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.11.022
arrangement of the NPs in a matrix [13–15]. When a stimuli responsive polymer layer is physically or chemically attached to the NP’s surface, not only a steric stability is acquired for the NPs, but also the optical properties of the NPs can be controlled to some extent by applying different external stimuli including pH [16], solvent polarity [17], electrical field [18], light irradiation [19], temperature [20], and the like. One important stimuli responsive polymer is the thermally responsive poly(N -isopropylacrylamide) (PNIPAM), which exhibits a low critical solution temperature (LCST) at ∼32 ◦ C [20,21]. Below LCST, PNIPAM is hydrophilic and soluble in aqueous solution, but upon raising the temperature above LCST, the polymer becomes hydrophobic and insoluble, and aggregates in solution. At a molecular level, PNIPAM chain changes its conformation from a disordered, random coil to an ordered, collapsed globule. For PNIPAM-stabilized noble metal NPs, any change in polymer conformation will have effects on the stability and optical properties of the NPs. For exam-
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ple, when the phase transition results in a collapsed protecting polymer layer, coagulation, flocculation or even aggregation between NPs can take place [22]. Under such conditions the coupled plasmon absorbance, which comes from the close contact of optically absorbing NPs, will be observed [23]. The coupled surface plasmon absorbance is the fundament of optical/colorimetric sensors based on noble metal NPs [24–28]. Here we report on the thermo-controllable optical and electrochemical properties of PNIPAM-stabilized Ag NPs. The Ag NPs were synthesized by NaBH4 reduction of silver ions in the presence of the polymers. Different techniques including UV–vis spectroscopy, dynamic light scattering, transmission electron microscopy (TEM) and cyclic voltammeter (CV) were employed to characterize the PNIPAM-Ag NPs as a function of the thermal phase transition of PNIPAM. 2. Materials and methods 2.1. Chemicals and procedure Silver nitrate (AgNO3 , 99.9%), sodium borohydride (NaBH4 , 99%), and poly(N -isopropylacrylamide) (PNIPAM, Mn = 20,000–25,000) were purchased from Aldrich and used as received. The PNIPAM-stabilized Ag NPs were synthesized as follows: (i) 1 ml AgNO3 (25 mM) and 2 or 5 ml of PNIPAM (25 mM of repeating NIPAM units) solutions were thoroughly mixed in a 500 ml beaker containing 200 ml vigorously stirred water, (ii) 1 ml fresh NaBH4 solution (25 mM) was then slowly added into the mixed solution. The mixed solution immediately turned into yellow upon the addition of NaBH4 , indicating the rapid formation of Ag NPs. The resultant Ag NPs were left for 3 days at room temperature to let the residual NaBH4 decompose before any characterization was took place. 2.2. Instrumentation Optical absorption spectra of the Ag NPs were recorded by using a Cary 300 UV–vis spectrophotometer equipped with a temperature control unit. Size and shape of the Ag NPs were investigated by bright-field transmission electron microscopy (TEM, Hitachi HF 2000) operated at an acceleration voltage of 200 kV. TEM samples were prepared by mounting a droplet of the Ag NPs solution of interest on a carbon-coated copper grid (Cu-400CK, Pacific Grid-Tech, USA). The hydrodynamic radius of the PNIPAM-stabilized Ag NPs was measured as a function of temperature by dynamic light scattering (DLS). The DLS measurements were performed at the scattering angle of 90◦ using an ALV/CGS-3 compact goniometer (ALV, Langen, Germany) equipped with a cuvette rotation/translation unit (CRTU) and a He–Ne laser (22 mW, wavelength λ = 632.8 nm). Temperature control was achieved by an external thermostat with an accuracy of 0.01 ◦ C. To reach thermal equilibrium, the sample was kept at each temperature for at least 30 min before any data acquisition was performed. The hydrodynamic radius of the particles was then obtained by an inverse Laplace transform of the autocorrelation function (CONTIN analysis) with the integrated ALV software package. Cyclic
voltammetry experiments were carried out with a home-built potentiostat in air. A conventional three-electrode system was used throughout. All potentials were recorded and reported versus an Ag/AgCl/3 M KCl reference electrode. The working electrode was a bare carbon disk electrode or a carbon disk electrode coated with a dry PNIPAM-Ag NPs composite film (Φ = 1 mm). 3. Results and discussion 3.1. Optical absorption and morphology of PNIPAM-stabilized Ag NPs The solid curve in Fig. 1 shows the UV–vis spectrum recorded at 25 ◦ C from the PNIPAM-stabilized Ag NPs with an Ag:NIPAM molar ratio of 1:5 in aqueous solution. The wavelength of maximum absorbance (λmax ) and the full band width at half maximum (FWHM) are 394 and 45 nm, respectively. For the Ag NPs solutions with Ag:NIPAM molar ratio of 1:2 (Fig. 1, dashed curve), a red shift in the plasmon resonance absorbance to 397 nm along with a broadening of the FWHM to 63 nm were observed. These results indicate a larger size and wider size distribution for Ag NPs with less protecting polymers. Similar results have been reported for silver nanoparticles prepared in the interior of microgels [5,29,30] or in the solutions of unimolecular micelles or core–shell colloidal particles [5,31]. Representative TEM images (Fig. 2) of the PNIPAM-stabilized Ag NPs reveal that the obtained Ag NPs are spherical in shape and reasonably uniform in size. The mean radii of Ag NPs in Figs. 2A and 2B are 16.2 ± 3.3 and 19.6 ± 4.5 nm, respectively. Even in dried state, no aggregation of Ag NPs was observed, suggesting that the Ag NPs are stable and uniformly distributed in the aqueous solution. Although the micrographs taken by TEM refer to the dried state in which the PNIPAM chains are totally shrunken, it can be seen clearly from Fig. 2 that the Ag NPs are quite evenly distributed within the matrix of a continuous PNIPAM film. This observation is mainly due to the presence of excess PNIPAM chains. No centrifuging was performed in current experiments to purify the Ag NPs since the
Fig. 1. Normalized optical absorption spectra of the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 (solid curve) and 1:2 (dashed curve) measured at 25 ◦ C. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. The pH value of the solution is ca. 5.5.
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Fig. 2. TEM micrographs of the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 (A) and 1:2 (B). The average diameters of the Ag NPs in (A) and (B) are 16.2 ± 3.3 and 19.6 ± 4.5 nm, respectively.
PNIPAM solutions used are very dilute. However, the excess PNIPAM chains can still form a thin dry film during the sample preparation for TEM measurements. Upon heating the Ag NPs solutions to a temperature above LCST of PNIPAM, a volume phase transition will take place, which may affect the stability and optical properties of the Ag NPs. This process was followed by UV–vis spectroscopy at different temperatures. Fig. 3 shows the resulting absorption spectra of the Ag NPs below and above the LCST of PNIPAM. The plasmon peaks of the Ag NPs are originally centered at 394 nm (Ag:NIPAM 1:5) and 397 nm (Ag:NIPAM 1:2) at 25 ◦ C. When the solution temperature was increased to 45 ◦ C, red shifts of the plasmon peaks to 404 (Ag:NIPAM 1:5) and 406 (Ag:NIPAM 1:2) nm were observed. It should be noted that this transition is fully reversible upon cooling the colloidal solution to 25 ◦ C. Similar results have been reported by Shan et al. [32], where Au NPs protected with polystyrene and PNIPAM showed a red shift of the absorption maximum wavelength with increasing temperature, i.e., 568 nm at 20 ◦ C and 596 nm at 30 ◦ C. A red shift of ∼10 nm was also observed for silver nanoparticles prepared in the solutions of unimolecular micelles with PNIPAM corona [31], or in the solution of pure PNIPAM [33], or in the solutions of the core–shell colloidal particles with PNIPAM shell when heating up the systems above the LCST of PNIPAM [5]. Furthermore, there is no new absorbance band at longer wavelengths and even no obvious increase in absorption intensity from the UV–vis spectra measured at 45 ◦ C. As
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Fig. 3. Normalized optical absorption spectra of the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 (A) and 1:2 (B) at 25 ◦ C (dashed curve) and 45 ◦ C (solid curve). The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. The pH value of the solution is ca. 5.5.
we know, the obtained Ag NPs are mainly sterical-stabilized by the PNIPAM chains. Several groups [34,35] suggested the complexation of the silver ions by the nitrogen atoms of the PNIPAM, which may lead to the strong adsorption of PNIPAM onto the Ag NP surface and the formation of the stericalprotecting layer. The presence of hydrophobic groups in water has two opposite effects on the stability of the Ag NPs: [36] (1) hydrophobic hydration in which the hydrophobic group is surrounded by water molecules with cage-like structure, and (2) hydrophobic interaction. The former effect tends to stabilize the Ag NPs, while the latter one will induce aggregation. Increasing temperature above LCST will impair the hydrophobic hydration and enhance the hydrophobic interaction inside the PNIPAM coils because the total number of water molecules around the hydrophobic groups is reduced. No strongly coupled plasmon absorption at longer wavelengths was observed, which may suggest that the attractive hydrophobic interactions of PNIPAM chains induced by the temperature increase are not strong enough to produce large aggregation because many hydrophobic groups would still be surrounded by water molecules. For isolated noble metal NPs with spherical shape, the wavelength of the resonance absorbance maximum depends on particle size, surface-adsorbed species and dielectric medium surrounding the Ag NPs [37]. A red shift of the resonance absorbance can be attributed to the increase in particle size, or to the increase in the refractive index of the medium [38,39]. It has been reported that the PNIPAM membrane surface is hydrophilic as the amide groups hydrate at lower temperatures,
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while the polymer becomes packed with propyl groups, i.e., alkane-like, at higher temperatures [40,41]. Water contact angle values for a PNIPAM-coated surface below its LCST typically range between 45◦ and 55◦ , whereas the water contact angle rises to approximately 90◦ at temperatures above LCST, which is indeed an alkane-like surface [42]. Therefore, the PNIPAM chains tend to accumulate on the surface of the Ag NPs at temperatures above LCST when the thermal phase transition of PNIPAM occurs. Since the refractive index of PNIPAM is much higher than that of water, the restoring force for the conduction band electrons within the Ag NPs is reduced, so that the surface plasmon absorbance shifts to a lower frequency [13]. It should be noted here that increasing the solution temperature can also change the properties of the solvent (volume expansion and refractive index change) and the NPs (metal expansion and resistivity change) [15]. For the silver nanoparticles prepared in the solutions of unimolecular micelles with PNIPAM corona or in the solutions of the core–shell colloidal particles with PNIPAM shell, Lu et al. [5] and Xu et al. [31] suggested two contributions to the red shift of plasmon band at higher temperature. These are the decrease of the relative distances between neighboring Ag nanoparticles due to the shrinkage of the PNIPAM shell and the increase of the refractive index of the PNIPAM chains surrounding the Ag nanoparticles at higher temperature. Because the temperature range under investigation is relatively narrow (25–45 ◦ C) and the most significant change in our system is the phase transition of the thermoresponsive PNIPAM, we attribute our observations mainly to the change of the local dielectric environment experienced by Ag NPs. Morones and Frey [33] showed that a dense PNIPAM shell induces a refractive index change of about 0.1 for Ag NPs, which results in a red shift of the resonance absorbance for PNIPAM-stabilized Ag NPs above the LCST. In order to know whether the PNIPAM-stabilized Ag NPs form aggregates in an aqueous medium at 45 ◦ C, we seek help from the light scattering technique because of its much higher sensitivity to the formation of aggregates than that of UV–vis spectroscopy based on light extinction (absorption
Fig. 4. Temperature dependence of the average hydrodynamic radius of the pure PNIPAM (5 g/L) and the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 and 1:2. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. The pH value of the solution is ca. 5.5.
+ scattering). Fig. 4 shows the hydrodynamic radii of pure PNIPAM dissolved in water and of PNIPAM-stabilized Ag NPs as a function of the solution temperature. Dynamic light scattering measurements of Ag NPs at different temperatures indicate that the PNIPAM chains still show reversible thermosensitivity in the presence of Ag NPs, including shrinking and reswelling as a function of temperature. But the presence of Ag NPs has effects on the volume transition of PNIPAM. The PNIPAM-stabilized Ag NPs show a broadened volume transition, which might be attributed to the strong interaction between the PNIPAM chain and the Ag NPs surfaces. The strong adsorption of PNIPAM on the Ag NP surface may be due to strong localization of Ag within the polymer chains, which is most probably caused by a coordination of silver with the nitrogen atoms of the PNIPAM [35]. This strong interaction partially slows down the collapsing process of PNIPAM at higher temperature [43]. The temperature where PNIPAM chains start to collapse presents the volume transition temperature of the PNIPAM. It can then be seen from Fig. 4 that the volume transition temperature of the PNIPAM-stabilized Ag NPs is slightly smaller than that of PNIPAM homopolymer. The hydrodynamic radius (Rh ) of pure PNIPAM shows the typical behavior of a thermal responsive polymer with a LCST. A clear LCST of PNIPAM around 33 ◦ C was observed here. For the PNIPAMstabilized Ag NPs, Rh first decreases with increasing temperature below the LCST of PNIPAM. The minimum Rh is 23 and 32 nm for Ag NPs with Ag:NIPAM molar ratio of 1:5 and 1:2, respectively, which are close to the mean size of Ag NPs obtained by TEM (cf. Fig. 2). Note that the concentration of pure PNIPAM for DLS measurements is 5 g/L, while the concentrations of PNIPAM are 0.0283 and 0.0708 g/L for Ag NPs with Ag:NIPAM molar ratio of 1:2 and 1:5, respectively. We tried to perform measurements on pure PNIPAM solution with concentration of 0.0708 g/L, no good scattering signal can be obtained. Consistent with the TEM results, the DLS measurements indicate that the PNIPAM chains are absorbed on the Ag NPs and stabilize them in an aqueous solution. It can be seen that the Rh of the Ag NPs with Ag:NIPAM molar ratio of 1:5 is smaller than that of Ag NPs with Ag:NIPAM molar ratio of 1:2, which is consistent with the TEM results. When the temperature increases above LCST, Rh of the Ag NPs increases slightly. This may be due to the hydrophobic interaction of the collapsed PNIPAM chains, which will lead to aggregation of the Ag NPs. More PNIPAM chains on Ag NPs are, larger aggregates may be formed. As a result, the Rh of the Ag NPs with Ag:NIPAM molar ratio of 1:5 is larger than that of Ag NPs with Ag:NIPAM molar ratio of 1:2 at temperature above the LCST. Since the aqueous solution of the PNIPAM-stabilized Ag NPs is still transparent even at 45 ◦ C, a possible explanation is that only small aggregates of Ag NPs were formed. This explanation is also supported by the UV–vis results. If strong aggregation occurs for Ag NPs at higher temperature, strong red shift of surface plasmon band should be observed by UV–vis spectra. However, only slightly red shift was observed when heating up the PNIPAM-stabilized Ag NPs above the LCST of PNIPAM (cf. Fig. 3). The results obtained here are close to those reported in literature. Lu et al. [5] investigated the thermal behavior of
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Ag NPs prepared in the solution of thermosensitive core–shell particles with the PS core and PNIPAM shell. They found the Ag NPs mainly locate in the PNIPAM shell due to the complexation of silver ions by the nitrogen atoms of the PNIPAM. No apparent aggregation is observed by DLS at higher temperature up to 45 ◦ C. Xu et al. [31] studied the in-situ formation of Ag NPs in the present of unimolecular micelles with PNIPAM corona. The Ag NPs were found to be immobilized inside the PNIPAM shell of the unimolecular micelles due to the quite strong complexation of Ag nanoparticles to the PNIPAM shell. Again, no apparent aggregation was observed when heating the system up to 40 ◦ C. The stability of the Ag NPs at higher temperature may benefit from the advanced physical properties of the protecting PNIPAM layers. Kujawa et al. [44] have concluded that the intermolecular hydrogen bonds formed between PNIPAM chains in collapsed state, the viscoelastic effect, and the partial vitrification of collapsed PNIPAM as well as the contribution of the end groups lead to the stability of collapsed PNIPAM at higher temperature. Depending on the initiator, the end groups of the PNIPA can be charged or neutral. For the charged end groups, the electrostatic interactions may additionally enhance the stability of the Ag NPs at higher temperature. Although the type of end groups in the PNIPAM used here is unknown, the other three factors may contribute to the stability of the PNIPAMcapped Ag NPs at higher temperature. To assess the change of the morphology of Ag NPs caused by the phase transition of PNIPAM we again studied the Ag NPs from the respective colloidal solutions at 45 ◦ C. Representative TEM micrographs (see Fig. S1 in Supplementary material) show the size and size distribution of the PNIPAMstabilized Ag NPs. The TEM images support the hypothesis that the Ag NPs do not form large aggregates when the phase transition of the PNIPAM occurs. Notice that the PNIPAM film on the TEM copper grid is not continuous when the colloidal solution was dried at 45 ◦ C. At the same time, most of the Ag NPs within the PNIPAM film are still isolated. But when the amount of the PNIPAM in the Ag NPs solution was decreased (such as Ag:NIPAM 1:2), somewhat more aggregates were observed by TEM (Fig. S1B). 3.2. Electrochemistry of PNIPAM-stabilized Ag NPs In order to investigate the electrochemical properties of PNIPAM-stabilized Ag NPs, we took the Ag NPs as the redox probe and perform cyclic voltammetry (CV) by oxidizing the protected Ag NPs as a function of the volume phase transition of the PNIPAM in air. We present only the results of the Ag NPs solutions with Ag:NIPAM molar ratio of 1:5, because the Ag NPs solutions with Ag:NIPAM molar ratio of 1:2 demonstrated very similar electrochemical behaviors. Fig. 5 shows the CVs of the PNIPAM-stabilized Ag NPs at 25 ◦ C (dashed curve) and 45 ◦ C (solid curve) in 25 mM NaNO3 solution. Higher electrolyte concentration will induce irreversible aggregation of the Ag NPs. At 25 ◦ C only the background current arising from charging of the electric double layer was observed in the potential range of 0.0–0.4 V. The CV at 45 ◦ C is markedly differ-
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Fig. 5. Cyclic voltammograms of a carbon electrode in an electrolyte containing 25 mM NaNO3 and the PNIPAM-stabilized Ag NPs at 25 ◦ C (dashed curve) and 45 ◦ C (solid curve) in air. The pH value of the solution is ca. 5.5. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. Scan rate: 50 mV/s.
Fig. 6. The effect of the negative reversal potentials of cyclic voltammetry on the redox accessibility of Ag NPs stabilized by PNIPAM in 25 mM NaNO3 solution at 25 ◦ C in air. The pH value of the solution is ca. 5.5. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. Scan rate: 50 mV/s.
ent. Both a well-defined oxidation peak and a broad reduction peak were observed, indicating that the oxidation of Ag NPs and the subsequent reduction of the stripped Ag+ ions are irreversible [45]. The results of CVs indicate that the PNIPAM protecting layer inhibits the redox process of the Ag NPs when the polymer chains are in their hydrated state below their LCST. The Ag NPs are electrochemically accessible above LCST, suggesting that the PNIPAM in its collapsed state cannot prevent Ag NPs from being close to the electrode surface and being oxidized there. Comparing the peak areas of the oxidation and reduction peaks, it can be concluded that most of the stripped Ag+ ions diffuse into the bulk solution and cannot be effectively deposited on the electrode surface. Besides temperature, negative or positive polarization of the electrode to extreme potentials also has effects on the redox chemistry of the Ag NPs. However the effects from electrode polarization are minor compared with those from the change of temperature. Fig. 6 shows the CVs of the PNIPAM-stabilized Ag NPs at different negative reversal potentials at 25 ◦ C. When the negative reverse potential in the CVs is lowered to −0.3 V,
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Fig. 7. Cyclic voltammograms of a carbon electrode coated with a thin film of the PNIPAM-Ag NP composites in 0.1 M NaNO3 solutions at 25 ◦ C (solid curve) and 45 ◦ C (dashed curve) in air. The pH value of the solution is 5.5. Scan rate: 50 mV/s.
a weak current corresponding to the oxidation of Ag NPs becomes observable and the subsequent scans lead to enhanced oxidizing current. But the total charges involved in the oxidation are less than 5% of those involved in the oxidation at 45 ◦ C, even when the negative reverse potential was set to −0.5 V. To test the potential usage of PNIPAM-stabilized Ag NPs as electrode coating material, the CV of PNIPAM-Ag NP composites deposited on the electrode surface in a dry state was also performed in air. Fig. 7 shows the CVs of a composite film-coated carbon electrode in 0.1 M NaNO3 solution at 25 ◦ C (solid curve) and 45 ◦ C (dashed curve). For the measurements at 25 ◦ C, a small droplet of the PNIPAM-Ag NPs solution was applied to the electrode surface and dried at room temperature; while for the measurements at 45 ◦ C, droplets from colloidal solution preheated to 45 ◦ C were used and the coated electrode was dried in a thermostatic oven at 45 ◦ C. The redox label, the embedded Ag NPs, exhibits well-defined oxidation and reduction peaks at 25 ◦ C, a condition at which the polymer is hydrated; while at 45 ◦ C the response of Ag NPs is completely suppressed. Thus the polymer shrinking fully insulates the electrode surface toward the electrolyte ions penetrating into the film, which is necessary for the charge compensation when Ag NPs are, if possible, oxidized into Ag+ ions. At 25 ◦ C, the PNIPAM-Ag NPs composite film is prone to desorbing from the electrode surface, so that the redox currents become weak after prolonged potential cycling, as has already been seen in Fig. 7 (solid curves): the peak current of the fifth cycle is decreased to 60% of the first cycle. For the electrode dried and measured at 45 ◦ C, when we decrease the electrolyte solution temperature down to 25 ◦ C, the currents corresponding to the redox of Ag NPs are observed, as expected. From Figs. 5 and 7, the electrochemical behaviors of the PNIPAM-Ag NPs solution seem to contrast with those of the PNIPAM-Ag NPs composite films. However, if we look closely, the results are consistent and understandable. For any electrochemical signal, the PNIPAM-Ag NPs have to be in close contact with the electrode surface. In the case of colloidal solutions, the electrochemical signal comes from the Ag nanoparticles adsorbed onto the electrode surface. At lower temperature
of 25 ◦ C, the hydrated PNIPAM layers protect the Ag NPs and prevent them from being absorbed onto the electrode surface. While at 45 ◦ C, the collapsed PNIPAM layers make the Ag NPs easier to be absorbed onto the electrode surface. This is also the reason why the electrochemical signal can be observed for the dilute silver nanoparticle solutions at 45 ◦ C. In the case of polyacrylate stabilized Ag NPs reported by Ung et al. [46], the protecting polymer layers inhibit the nanoparticles away from being absorbed on the electrode surface so that the electrochemical signal can only be observed for very concentrated colloidal solution. However, for the PNIPAM-Ag NPs composite films prepared at 25 ◦ C, the surface concentration of Ag NPs is quite large since the films is deposited on the electrode surface and the Ag NPs have contact well with the electrode. Thus, good electrochemical signal can be obtained. Contrary, for the films prepared at 45 ◦ C, the collapsed PNIPAM layers insolate the Ag NPs from the electrode surface. Therefore, no signal was observed when measuring at 45 ◦ C. Instead, when the measurements were performed at 25 ◦ C for the films prepared at 45 ◦ C, the PNIPAM layers become hydrated again and the Ag NPs regain good contact with the electrode surface so that the electrochemical signal was observed again. 4. Summary Thermo-responsive PNIPAM-stabilized Ag NPs were successfully synthesized by simply reducing the silver salt in the PNIPAM aqueous solution. The obtained PNIPAM-stabilized Ag NPs show tunable optical properties, which is fully reversible dependence on the phase transition of PNIPAM. Above the LCST of PNIPAM, the collapse of the PNIPAM chains increases the refractive index of the environments experienced by Ag NPs, resulting in the red shift of the surface plasmon resonance band. Interestingly, it was found that the PNIPAM chains are able to stabilize Ag NPs not only with the extended coil conformation at lower temperature but also with the collapsed globule conformation above the LCST. The electrochemical properties of PNIPAM-stabilized Ag NPs also strongly depend on the temperature. The silver particles only show electrochemical activity when the PNIPAM chains collapse. These PNIPAM-stabilized Ag NPs may have potential applications as electrode coating materials, for which the electrochemical properties could be switched by the environmental temperature. Acknowledgments Z. Peng acknowledges the support by the Alexander von Humboldt Foundation. B. Du thanks the partially financial support by the National Natural Science Foundation of China (No. 20604022) and Zhejiang Provincial Natural Science Foundation of China (No. Y406029). Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2007.11.022.
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Thermoresponsive polymer-stabilized silver nanoparticles Limin Guo a,1 , Jingjing Nie b , Binyang Du c,∗ , Zhangquan Peng a,∗,1 , Bernd Tesche d , Karl Kleinermanns a,∗ a Institut für Physikalische Chemie, Heinrich-Heine-Universität Düsseldorf, 40225 Düsseldorf, Germany b Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China c MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, PR China d Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany
Received 10 September 2007; accepted 19 November 2007 Available online 24 November 2007
Abstract Silver nanoparticles (Ag NPs) stabilized by a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), have been synthesized by the reduction of silver ions with NaBH4 in aqueous solutions. The obtained Ag NPs are very stable at room temperature due to the extended coil conformation of the PNIPAM chain at temperatures below its volume phase transition temperature (∼32 ◦ C). At higher temperatures (such as 45 ◦ C) above the phase transition of PNIPAM, only minute aggregation between Ag NPs was observed, showing that the collapsed PNIPAM chains still retain the ability to stabilize Ag NPs. The PNIPAM-stabilized Ag NPs were then characterized as a function of the thermal phase transition of PNIPAM by UV–vis spectroscopy, dynamic light scattering, transmission electron microscopy, and cyclic voltammeter. Consistent results were obtained showing that the phase transition of PNIPAM has some effect on the optical properties of Ag NPs. Switchable electrochemical response of the PNIPAM-stabilized Ag NPs triggered by temperature change was observed. © 2007 Elsevier Inc. All rights reserved. Keywords: Silver nanoparticles; Thermoresponsive polymer; Optical property; Electrochemistry
1. Introduction Surface modification of noble metal nanoparticles (NPs) with stimuli responsive polymers, their optical properties and the assemblies of such modified NPs are subjects of extensive research efforts [1–8]. Noble metal NPs (such as Au and Ag) often exhibit strong plasmon resonance absorption in the visible spectrum, which depends on the size, shape and surface chemistry of the NPs [9–12]. Furthermore, while the composition of the NPs may be held constant, its plasmon resonance absorption maximum could be shifted hundreds of nanometers by changing its shape, orientation in the incident light, or the * Corresponding authors. Fax: +86 571 87952400.
E-mail addresses: [email protected] (B. Du), [email protected] (Z. Peng), [email protected] (K. Kleinermanns). 1 Present address: Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark. 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.11.022
arrangement of the NPs in a matrix [13–15]. When a stimuli responsive polymer layer is physically or chemically attached to the NP’s surface, not only a steric stability is acquired for the NPs, but also the optical properties of the NPs can be controlled to some extent by applying different external stimuli including pH [16], solvent polarity [17], electrical field [18], light irradiation [19], temperature [20], and the like. One important stimuli responsive polymer is the thermally responsive poly(N -isopropylacrylamide) (PNIPAM), which exhibits a low critical solution temperature (LCST) at ∼32 ◦ C [20,21]. Below LCST, PNIPAM is hydrophilic and soluble in aqueous solution, but upon raising the temperature above LCST, the polymer becomes hydrophobic and insoluble, and aggregates in solution. At a molecular level, PNIPAM chain changes its conformation from a disordered, random coil to an ordered, collapsed globule. For PNIPAM-stabilized noble metal NPs, any change in polymer conformation will have effects on the stability and optical properties of the NPs. For exam-
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ple, when the phase transition results in a collapsed protecting polymer layer, coagulation, flocculation or even aggregation between NPs can take place [22]. Under such conditions the coupled plasmon absorbance, which comes from the close contact of optically absorbing NPs, will be observed [23]. The coupled surface plasmon absorbance is the fundament of optical/colorimetric sensors based on noble metal NPs [24–28]. Here we report on the thermo-controllable optical and electrochemical properties of PNIPAM-stabilized Ag NPs. The Ag NPs were synthesized by NaBH4 reduction of silver ions in the presence of the polymers. Different techniques including UV–vis spectroscopy, dynamic light scattering, transmission electron microscopy (TEM) and cyclic voltammeter (CV) were employed to characterize the PNIPAM-Ag NPs as a function of the thermal phase transition of PNIPAM. 2. Materials and methods 2.1. Chemicals and procedure Silver nitrate (AgNO3 , 99.9%), sodium borohydride (NaBH4 , 99%), and poly(N -isopropylacrylamide) (PNIPAM, Mn = 20,000–25,000) were purchased from Aldrich and used as received. The PNIPAM-stabilized Ag NPs were synthesized as follows: (i) 1 ml AgNO3 (25 mM) and 2 or 5 ml of PNIPAM (25 mM of repeating NIPAM units) solutions were thoroughly mixed in a 500 ml beaker containing 200 ml vigorously stirred water, (ii) 1 ml fresh NaBH4 solution (25 mM) was then slowly added into the mixed solution. The mixed solution immediately turned into yellow upon the addition of NaBH4 , indicating the rapid formation of Ag NPs. The resultant Ag NPs were left for 3 days at room temperature to let the residual NaBH4 decompose before any characterization was took place. 2.2. Instrumentation Optical absorption spectra of the Ag NPs were recorded by using a Cary 300 UV–vis spectrophotometer equipped with a temperature control unit. Size and shape of the Ag NPs were investigated by bright-field transmission electron microscopy (TEM, Hitachi HF 2000) operated at an acceleration voltage of 200 kV. TEM samples were prepared by mounting a droplet of the Ag NPs solution of interest on a carbon-coated copper grid (Cu-400CK, Pacific Grid-Tech, USA). The hydrodynamic radius of the PNIPAM-stabilized Ag NPs was measured as a function of temperature by dynamic light scattering (DLS). The DLS measurements were performed at the scattering angle of 90◦ using an ALV/CGS-3 compact goniometer (ALV, Langen, Germany) equipped with a cuvette rotation/translation unit (CRTU) and a He–Ne laser (22 mW, wavelength λ = 632.8 nm). Temperature control was achieved by an external thermostat with an accuracy of 0.01 ◦ C. To reach thermal equilibrium, the sample was kept at each temperature for at least 30 min before any data acquisition was performed. The hydrodynamic radius of the particles was then obtained by an inverse Laplace transform of the autocorrelation function (CONTIN analysis) with the integrated ALV software package. Cyclic
voltammetry experiments were carried out with a home-built potentiostat in air. A conventional three-electrode system was used throughout. All potentials were recorded and reported versus an Ag/AgCl/3 M KCl reference electrode. The working electrode was a bare carbon disk electrode or a carbon disk electrode coated with a dry PNIPAM-Ag NPs composite film (Φ = 1 mm). 3. Results and discussion 3.1. Optical absorption and morphology of PNIPAM-stabilized Ag NPs The solid curve in Fig. 1 shows the UV–vis spectrum recorded at 25 ◦ C from the PNIPAM-stabilized Ag NPs with an Ag:NIPAM molar ratio of 1:5 in aqueous solution. The wavelength of maximum absorbance (λmax ) and the full band width at half maximum (FWHM) are 394 and 45 nm, respectively. For the Ag NPs solutions with Ag:NIPAM molar ratio of 1:2 (Fig. 1, dashed curve), a red shift in the plasmon resonance absorbance to 397 nm along with a broadening of the FWHM to 63 nm were observed. These results indicate a larger size and wider size distribution for Ag NPs with less protecting polymers. Similar results have been reported for silver nanoparticles prepared in the interior of microgels [5,29,30] or in the solutions of unimolecular micelles or core–shell colloidal particles [5,31]. Representative TEM images (Fig. 2) of the PNIPAM-stabilized Ag NPs reveal that the obtained Ag NPs are spherical in shape and reasonably uniform in size. The mean radii of Ag NPs in Figs. 2A and 2B are 16.2 ± 3.3 and 19.6 ± 4.5 nm, respectively. Even in dried state, no aggregation of Ag NPs was observed, suggesting that the Ag NPs are stable and uniformly distributed in the aqueous solution. Although the micrographs taken by TEM refer to the dried state in which the PNIPAM chains are totally shrunken, it can be seen clearly from Fig. 2 that the Ag NPs are quite evenly distributed within the matrix of a continuous PNIPAM film. This observation is mainly due to the presence of excess PNIPAM chains. No centrifuging was performed in current experiments to purify the Ag NPs since the
Fig. 1. Normalized optical absorption spectra of the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 (solid curve) and 1:2 (dashed curve) measured at 25 ◦ C. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. The pH value of the solution is ca. 5.5.
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Fig. 2. TEM micrographs of the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 (A) and 1:2 (B). The average diameters of the Ag NPs in (A) and (B) are 16.2 ± 3.3 and 19.6 ± 4.5 nm, respectively.
PNIPAM solutions used are very dilute. However, the excess PNIPAM chains can still form a thin dry film during the sample preparation for TEM measurements. Upon heating the Ag NPs solutions to a temperature above LCST of PNIPAM, a volume phase transition will take place, which may affect the stability and optical properties of the Ag NPs. This process was followed by UV–vis spectroscopy at different temperatures. Fig. 3 shows the resulting absorption spectra of the Ag NPs below and above the LCST of PNIPAM. The plasmon peaks of the Ag NPs are originally centered at 394 nm (Ag:NIPAM 1:5) and 397 nm (Ag:NIPAM 1:2) at 25 ◦ C. When the solution temperature was increased to 45 ◦ C, red shifts of the plasmon peaks to 404 (Ag:NIPAM 1:5) and 406 (Ag:NIPAM 1:2) nm were observed. It should be noted that this transition is fully reversible upon cooling the colloidal solution to 25 ◦ C. Similar results have been reported by Shan et al. [32], where Au NPs protected with polystyrene and PNIPAM showed a red shift of the absorption maximum wavelength with increasing temperature, i.e., 568 nm at 20 ◦ C and 596 nm at 30 ◦ C. A red shift of ∼10 nm was also observed for silver nanoparticles prepared in the solutions of unimolecular micelles with PNIPAM corona [31], or in the solution of pure PNIPAM [33], or in the solutions of the core–shell colloidal particles with PNIPAM shell when heating up the systems above the LCST of PNIPAM [5]. Furthermore, there is no new absorbance band at longer wavelengths and even no obvious increase in absorption intensity from the UV–vis spectra measured at 45 ◦ C. As
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Fig. 3. Normalized optical absorption spectra of the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 (A) and 1:2 (B) at 25 ◦ C (dashed curve) and 45 ◦ C (solid curve). The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. The pH value of the solution is ca. 5.5.
we know, the obtained Ag NPs are mainly sterical-stabilized by the PNIPAM chains. Several groups [34,35] suggested the complexation of the silver ions by the nitrogen atoms of the PNIPAM, which may lead to the strong adsorption of PNIPAM onto the Ag NP surface and the formation of the stericalprotecting layer. The presence of hydrophobic groups in water has two opposite effects on the stability of the Ag NPs: [36] (1) hydrophobic hydration in which the hydrophobic group is surrounded by water molecules with cage-like structure, and (2) hydrophobic interaction. The former effect tends to stabilize the Ag NPs, while the latter one will induce aggregation. Increasing temperature above LCST will impair the hydrophobic hydration and enhance the hydrophobic interaction inside the PNIPAM coils because the total number of water molecules around the hydrophobic groups is reduced. No strongly coupled plasmon absorption at longer wavelengths was observed, which may suggest that the attractive hydrophobic interactions of PNIPAM chains induced by the temperature increase are not strong enough to produce large aggregation because many hydrophobic groups would still be surrounded by water molecules. For isolated noble metal NPs with spherical shape, the wavelength of the resonance absorbance maximum depends on particle size, surface-adsorbed species and dielectric medium surrounding the Ag NPs [37]. A red shift of the resonance absorbance can be attributed to the increase in particle size, or to the increase in the refractive index of the medium [38,39]. It has been reported that the PNIPAM membrane surface is hydrophilic as the amide groups hydrate at lower temperatures,
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while the polymer becomes packed with propyl groups, i.e., alkane-like, at higher temperatures [40,41]. Water contact angle values for a PNIPAM-coated surface below its LCST typically range between 45◦ and 55◦ , whereas the water contact angle rises to approximately 90◦ at temperatures above LCST, which is indeed an alkane-like surface [42]. Therefore, the PNIPAM chains tend to accumulate on the surface of the Ag NPs at temperatures above LCST when the thermal phase transition of PNIPAM occurs. Since the refractive index of PNIPAM is much higher than that of water, the restoring force for the conduction band electrons within the Ag NPs is reduced, so that the surface plasmon absorbance shifts to a lower frequency [13]. It should be noted here that increasing the solution temperature can also change the properties of the solvent (volume expansion and refractive index change) and the NPs (metal expansion and resistivity change) [15]. For the silver nanoparticles prepared in the solutions of unimolecular micelles with PNIPAM corona or in the solutions of the core–shell colloidal particles with PNIPAM shell, Lu et al. [5] and Xu et al. [31] suggested two contributions to the red shift of plasmon band at higher temperature. These are the decrease of the relative distances between neighboring Ag nanoparticles due to the shrinkage of the PNIPAM shell and the increase of the refractive index of the PNIPAM chains surrounding the Ag nanoparticles at higher temperature. Because the temperature range under investigation is relatively narrow (25–45 ◦ C) and the most significant change in our system is the phase transition of the thermoresponsive PNIPAM, we attribute our observations mainly to the change of the local dielectric environment experienced by Ag NPs. Morones and Frey [33] showed that a dense PNIPAM shell induces a refractive index change of about 0.1 for Ag NPs, which results in a red shift of the resonance absorbance for PNIPAM-stabilized Ag NPs above the LCST. In order to know whether the PNIPAM-stabilized Ag NPs form aggregates in an aqueous medium at 45 ◦ C, we seek help from the light scattering technique because of its much higher sensitivity to the formation of aggregates than that of UV–vis spectroscopy based on light extinction (absorption
Fig. 4. Temperature dependence of the average hydrodynamic radius of the pure PNIPAM (5 g/L) and the PNIPAM-stabilized Ag NPs with Ag:NIPAM molar ratio of 1:5 and 1:2. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. The pH value of the solution is ca. 5.5.
+ scattering). Fig. 4 shows the hydrodynamic radii of pure PNIPAM dissolved in water and of PNIPAM-stabilized Ag NPs as a function of the solution temperature. Dynamic light scattering measurements of Ag NPs at different temperatures indicate that the PNIPAM chains still show reversible thermosensitivity in the presence of Ag NPs, including shrinking and reswelling as a function of temperature. But the presence of Ag NPs has effects on the volume transition of PNIPAM. The PNIPAM-stabilized Ag NPs show a broadened volume transition, which might be attributed to the strong interaction between the PNIPAM chain and the Ag NPs surfaces. The strong adsorption of PNIPAM on the Ag NP surface may be due to strong localization of Ag within the polymer chains, which is most probably caused by a coordination of silver with the nitrogen atoms of the PNIPAM [35]. This strong interaction partially slows down the collapsing process of PNIPAM at higher temperature [43]. The temperature where PNIPAM chains start to collapse presents the volume transition temperature of the PNIPAM. It can then be seen from Fig. 4 that the volume transition temperature of the PNIPAM-stabilized Ag NPs is slightly smaller than that of PNIPAM homopolymer. The hydrodynamic radius (Rh ) of pure PNIPAM shows the typical behavior of a thermal responsive polymer with a LCST. A clear LCST of PNIPAM around 33 ◦ C was observed here. For the PNIPAMstabilized Ag NPs, Rh first decreases with increasing temperature below the LCST of PNIPAM. The minimum Rh is 23 and 32 nm for Ag NPs with Ag:NIPAM molar ratio of 1:5 and 1:2, respectively, which are close to the mean size of Ag NPs obtained by TEM (cf. Fig. 2). Note that the concentration of pure PNIPAM for DLS measurements is 5 g/L, while the concentrations of PNIPAM are 0.0283 and 0.0708 g/L for Ag NPs with Ag:NIPAM molar ratio of 1:2 and 1:5, respectively. We tried to perform measurements on pure PNIPAM solution with concentration of 0.0708 g/L, no good scattering signal can be obtained. Consistent with the TEM results, the DLS measurements indicate that the PNIPAM chains are absorbed on the Ag NPs and stabilize them in an aqueous solution. It can be seen that the Rh of the Ag NPs with Ag:NIPAM molar ratio of 1:5 is smaller than that of Ag NPs with Ag:NIPAM molar ratio of 1:2, which is consistent with the TEM results. When the temperature increases above LCST, Rh of the Ag NPs increases slightly. This may be due to the hydrophobic interaction of the collapsed PNIPAM chains, which will lead to aggregation of the Ag NPs. More PNIPAM chains on Ag NPs are, larger aggregates may be formed. As a result, the Rh of the Ag NPs with Ag:NIPAM molar ratio of 1:5 is larger than that of Ag NPs with Ag:NIPAM molar ratio of 1:2 at temperature above the LCST. Since the aqueous solution of the PNIPAM-stabilized Ag NPs is still transparent even at 45 ◦ C, a possible explanation is that only small aggregates of Ag NPs were formed. This explanation is also supported by the UV–vis results. If strong aggregation occurs for Ag NPs at higher temperature, strong red shift of surface plasmon band should be observed by UV–vis spectra. However, only slightly red shift was observed when heating up the PNIPAM-stabilized Ag NPs above the LCST of PNIPAM (cf. Fig. 3). The results obtained here are close to those reported in literature. Lu et al. [5] investigated the thermal behavior of
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Ag NPs prepared in the solution of thermosensitive core–shell particles with the PS core and PNIPAM shell. They found the Ag NPs mainly locate in the PNIPAM shell due to the complexation of silver ions by the nitrogen atoms of the PNIPAM. No apparent aggregation is observed by DLS at higher temperature up to 45 ◦ C. Xu et al. [31] studied the in-situ formation of Ag NPs in the present of unimolecular micelles with PNIPAM corona. The Ag NPs were found to be immobilized inside the PNIPAM shell of the unimolecular micelles due to the quite strong complexation of Ag nanoparticles to the PNIPAM shell. Again, no apparent aggregation was observed when heating the system up to 40 ◦ C. The stability of the Ag NPs at higher temperature may benefit from the advanced physical properties of the protecting PNIPAM layers. Kujawa et al. [44] have concluded that the intermolecular hydrogen bonds formed between PNIPAM chains in collapsed state, the viscoelastic effect, and the partial vitrification of collapsed PNIPAM as well as the contribution of the end groups lead to the stability of collapsed PNIPAM at higher temperature. Depending on the initiator, the end groups of the PNIPA can be charged or neutral. For the charged end groups, the electrostatic interactions may additionally enhance the stability of the Ag NPs at higher temperature. Although the type of end groups in the PNIPAM used here is unknown, the other three factors may contribute to the stability of the PNIPAMcapped Ag NPs at higher temperature. To assess the change of the morphology of Ag NPs caused by the phase transition of PNIPAM we again studied the Ag NPs from the respective colloidal solutions at 45 ◦ C. Representative TEM micrographs (see Fig. S1 in Supplementary material) show the size and size distribution of the PNIPAMstabilized Ag NPs. The TEM images support the hypothesis that the Ag NPs do not form large aggregates when the phase transition of the PNIPAM occurs. Notice that the PNIPAM film on the TEM copper grid is not continuous when the colloidal solution was dried at 45 ◦ C. At the same time, most of the Ag NPs within the PNIPAM film are still isolated. But when the amount of the PNIPAM in the Ag NPs solution was decreased (such as Ag:NIPAM 1:2), somewhat more aggregates were observed by TEM (Fig. S1B). 3.2. Electrochemistry of PNIPAM-stabilized Ag NPs In order to investigate the electrochemical properties of PNIPAM-stabilized Ag NPs, we took the Ag NPs as the redox probe and perform cyclic voltammetry (CV) by oxidizing the protected Ag NPs as a function of the volume phase transition of the PNIPAM in air. We present only the results of the Ag NPs solutions with Ag:NIPAM molar ratio of 1:5, because the Ag NPs solutions with Ag:NIPAM molar ratio of 1:2 demonstrated very similar electrochemical behaviors. Fig. 5 shows the CVs of the PNIPAM-stabilized Ag NPs at 25 ◦ C (dashed curve) and 45 ◦ C (solid curve) in 25 mM NaNO3 solution. Higher electrolyte concentration will induce irreversible aggregation of the Ag NPs. At 25 ◦ C only the background current arising from charging of the electric double layer was observed in the potential range of 0.0–0.4 V. The CV at 45 ◦ C is markedly differ-
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Fig. 5. Cyclic voltammograms of a carbon electrode in an electrolyte containing 25 mM NaNO3 and the PNIPAM-stabilized Ag NPs at 25 ◦ C (dashed curve) and 45 ◦ C (solid curve) in air. The pH value of the solution is ca. 5.5. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. Scan rate: 50 mV/s.
Fig. 6. The effect of the negative reversal potentials of cyclic voltammetry on the redox accessibility of Ag NPs stabilized by PNIPAM in 25 mM NaNO3 solution at 25 ◦ C in air. The pH value of the solution is ca. 5.5. The concentration of Ag NPs with regard to Ag atoms is 0.125 mM. Scan rate: 50 mV/s.
ent. Both a well-defined oxidation peak and a broad reduction peak were observed, indicating that the oxidation of Ag NPs and the subsequent reduction of the stripped Ag+ ions are irreversible [45]. The results of CVs indicate that the PNIPAM protecting layer inhibits the redox process of the Ag NPs when the polymer chains are in their hydrated state below their LCST. The Ag NPs are electrochemically accessible above LCST, suggesting that the PNIPAM in its collapsed state cannot prevent Ag NPs from being close to the electrode surface and being oxidized there. Comparing the peak areas of the oxidation and reduction peaks, it can be concluded that most of the stripped Ag+ ions diffuse into the bulk solution and cannot be effectively deposited on the electrode surface. Besides temperature, negative or positive polarization of the electrode to extreme potentials also has effects on the redox chemistry of the Ag NPs. However the effects from electrode polarization are minor compared with those from the change of temperature. Fig. 6 shows the CVs of the PNIPAM-stabilized Ag NPs at different negative reversal potentials at 25 ◦ C. When the negative reverse potential in the CVs is lowered to −0.3 V,
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Fig. 7. Cyclic voltammograms of a carbon electrode coated with a thin film of the PNIPAM-Ag NP composites in 0.1 M NaNO3 solutions at 25 ◦ C (solid curve) and 45 ◦ C (dashed curve) in air. The pH value of the solution is 5.5. Scan rate: 50 mV/s.
a weak current corresponding to the oxidation of Ag NPs becomes observable and the subsequent scans lead to enhanced oxidizing current. But the total charges involved in the oxidation are less than 5% of those involved in the oxidation at 45 ◦ C, even when the negative reverse potential was set to −0.5 V. To test the potential usage of PNIPAM-stabilized Ag NPs as electrode coating material, the CV of PNIPAM-Ag NP composites deposited on the electrode surface in a dry state was also performed in air. Fig. 7 shows the CVs of a composite film-coated carbon electrode in 0.1 M NaNO3 solution at 25 ◦ C (solid curve) and 45 ◦ C (dashed curve). For the measurements at 25 ◦ C, a small droplet of the PNIPAM-Ag NPs solution was applied to the electrode surface and dried at room temperature; while for the measurements at 45 ◦ C, droplets from colloidal solution preheated to 45 ◦ C were used and the coated electrode was dried in a thermostatic oven at 45 ◦ C. The redox label, the embedded Ag NPs, exhibits well-defined oxidation and reduction peaks at 25 ◦ C, a condition at which the polymer is hydrated; while at 45 ◦ C the response of Ag NPs is completely suppressed. Thus the polymer shrinking fully insulates the electrode surface toward the electrolyte ions penetrating into the film, which is necessary for the charge compensation when Ag NPs are, if possible, oxidized into Ag+ ions. At 25 ◦ C, the PNIPAM-Ag NPs composite film is prone to desorbing from the electrode surface, so that the redox currents become weak after prolonged potential cycling, as has already been seen in Fig. 7 (solid curves): the peak current of the fifth cycle is decreased to 60% of the first cycle. For the electrode dried and measured at 45 ◦ C, when we decrease the electrolyte solution temperature down to 25 ◦ C, the currents corresponding to the redox of Ag NPs are observed, as expected. From Figs. 5 and 7, the electrochemical behaviors of the PNIPAM-Ag NPs solution seem to contrast with those of the PNIPAM-Ag NPs composite films. However, if we look closely, the results are consistent and understandable. For any electrochemical signal, the PNIPAM-Ag NPs have to be in close contact with the electrode surface. In the case of colloidal solutions, the electrochemical signal comes from the Ag nanoparticles adsorbed onto the electrode surface. At lower temperature
of 25 ◦ C, the hydrated PNIPAM layers protect the Ag NPs and prevent them from being absorbed onto the electrode surface. While at 45 ◦ C, the collapsed PNIPAM layers make the Ag NPs easier to be absorbed onto the electrode surface. This is also the reason why the electrochemical signal can be observed for the dilute silver nanoparticle solutions at 45 ◦ C. In the case of polyacrylate stabilized Ag NPs reported by Ung et al. [46], the protecting polymer layers inhibit the nanoparticles away from being absorbed on the electrode surface so that the electrochemical signal can only be observed for very concentrated colloidal solution. However, for the PNIPAM-Ag NPs composite films prepared at 25 ◦ C, the surface concentration of Ag NPs is quite large since the films is deposited on the electrode surface and the Ag NPs have contact well with the electrode. Thus, good electrochemical signal can be obtained. Contrary, for the films prepared at 45 ◦ C, the collapsed PNIPAM layers insolate the Ag NPs from the electrode surface. Therefore, no signal was observed when measuring at 45 ◦ C. Instead, when the measurements were performed at 25 ◦ C for the films prepared at 45 ◦ C, the PNIPAM layers become hydrated again and the Ag NPs regain good contact with the electrode surface so that the electrochemical signal was observed again. 4. Summary Thermo-responsive PNIPAM-stabilized Ag NPs were successfully synthesized by simply reducing the silver salt in the PNIPAM aqueous solution. The obtained PNIPAM-stabilized Ag NPs show tunable optical properties, which is fully reversible dependence on the phase transition of PNIPAM. Above the LCST of PNIPAM, the collapse of the PNIPAM chains increases the refractive index of the environments experienced by Ag NPs, resulting in the red shift of the surface plasmon resonance band. Interestingly, it was found that the PNIPAM chains are able to stabilize Ag NPs not only with the extended coil conformation at lower temperature but also with the collapsed globule conformation above the LCST. The electrochemical properties of PNIPAM-stabilized Ag NPs also strongly depend on the temperature. The silver particles only show electrochemical activity when the PNIPAM chains collapse. These PNIPAM-stabilized Ag NPs may have potential applications as electrode coating materials, for which the electrochemical properties could be switched by the environmental temperature. Acknowledgments Z. Peng acknowledges the support by the Alexander von Humboldt Foundation. B. Du thanks the partially financial support by the National Natural Science Foundation of China (No. 20604022) and Zhejiang Provincial Natural Science Foundation of China (No. Y406029). Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.jcis.2007.11.022.
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