Article pubs.acs.org/Langmuir
Tuning the Transmittance of Colloidal Solution by Changing the Orientation of Ag Nanoplates in Ferrofluid Yiwu Mao, Jing Liu, and Jianping Ge* Department of Chemistry, Tongji University, Shanghai 200092, China S Supporting Information *
ABSTRACT: Ag nanoplates and Fe3O4 nanoparticle-based ferrofluids were utilized to fabricate a magnetic field controlled optic switch. The changing of light transmittance (LT) is caused by the rotation of Ag nanoplates, whose long axis always follows the orientation of external magnetic field to minimize the potential energy. The sensitivity of switching was optimized by choosing Ag nanoplates with appropriate size and concentration. The switching of transmission is proved to be fast and fully reversible. This phenomenon not only indicates an effective method to adjust the propagation of optical signals, but also reveals the possibility and great potential to develop magnetic controlled functional devices. ferrofluid,24,25 but overall further investigation of this system is urgently needed. In this work, a magnetic field controlled optic switch with fast and reversible tunability in transmittance was fabricated by encapsulation of Ag nanoplates in ferrofluid. A new magnetic assembly system and tuning strategy based on the magnetic interaction of nonmagnetic particles in submicrometer scale was reported. The tuning of transmittance is based on the rotation of Ag nanoplates when the magnetic field orientation changes, which will eventually minimize their potential energy in ferrofluid (Figure 1). The as-prepared optical switch presents enhanced sensitivity, a larger light transmittance alteration (Δ(LT)) as compared to that of pure ferrofluid.26 Appropriate size and concentration of Ag nanoplates as well as suitable magnetic field strength were investigated to optimize the sensitivity and stability. The switching of transmittance is accomplished in 300 ms, and it is reversible for many cycles. The magnetic optical switch not only provides a new method to adjust the propagation of optical signals, but also reveals the possibility and great potential to develop magnetic controlled functional devices.
1. INTRODUCTION Tuning of magnetic field is a powerful method to remotely manipulate or assemble colloidal particles in various fluids. Thanks to the included superparamagnetic materials, which have paramagnetic behavior but much higher magnetic susceptibility, the rheological, optical, and thermal properties of the colloidal solution can be quickly and reversibly adjusted by the external magnetic field. These superior characteristics render the magnetic fluids great potentials to be used for vacuum sealing,1,2 sensing,3,4 optical switch,5−8 photonic printing,9,10 and display unit.11 For instance, optical modulator, grating, filter, and logic devices were fabricated utilizing the tunable refractive index of ferrofluid in the presence or absence of magnetic field.5 A fast and high-resolution color printing was developed by combining the magnetic assembly of Fe3O4@ SiO2 colloids with synchronized photopolymerization using maskless lithography techniques.9 A display unit analogous to the liquid crystal display can be fabricated on the basis of the orientational tuning of aluminum flakes attached with magnetic nanoparticle in rotary magnetic field.12 In contrast to the great enthusiasm in assembly of magnetic particles within a broad size range,13−21 little attention was paid to the manipulation of nonmagnetic building blocks in magnetically continuous media. In fact, the latter system is very promising to bring more interesting applications due to the diverse selectivity of nonmagnetic particles. Although it has been theoretically proven that the magnetic force and interactions can be effective stimuli to control these particles,21,22 some difficulties in the fluid’s stability, the synthesis of compatible nonmagnetic objects, and the design of magnetically controlled devices still limit their rapid development in the current stage. There have been some attempts recently, such as the magnetic alignment of nanorod23 or the magnetic tunable display unit base on polymer beads in © 2012 American Chemical Society
2. EXPERIMENTAL SECTION Chemicals. Diethylene glycol (DEG, 99%), anhydrous iron(III) chloride (FeCl3, 98%), poly (acrylic acid) (PAA, MW = 1800), sodium citrate tribasic dihydrate (TSC, 99%), polyvinylpyrrolidone (PVP, Mw = 29 000), and L-ascorbic acid were obtained from Sigma-Aldrich. Hydrogen peroxide (H2O2, 30 wt %), ethanol, sodium hydroxide (NaOH, 99%), silver nitrate (AgNO 3 , 99+%), and sodium borohydride (NaBH4, 96%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial optical glue NOA61 was Received: June 11, 2012 Revised: August 7, 2012 Published: August 8, 2012 13112
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transmission spectra of the liquid film were measured by an Ocean Optics Maya 2000 Pro spectrometer, and unpolarized light source was used to provide incident light. It should be noted that the transmittance of samples with magnetic field orientation parallel to the light path was set as reference (LT = 100%) for the measurement of all samples in this work. Characterizations. The morphology and size of the Ag nanoplates were investigated by TEM images obtained from a JEOL JEM-2100 transmission electron microscope. The optical microscope images were taken on an Olympus BX51 M reflective-type optical microscope operated in dark-field mode. The strength of applied magnetic field was measured by a gauss meter.
3. RESULTS AND DISCUSSION The colloidal optical switch was fabricated by encapsulating the aqueous solution of Ag nanoplates and the ferrofluid in a glass cell with an average thickness of 500 μm, which works in an orientational tunable magnetic field. The aqueous ferrofluids are suspension of highly water-soluble Fe3O4 nanocrystals, which are prepared by a solvothermal reaction with poly(acrylic acid) as surfactant (Figure S1).27 Ag nanoplates with different sizes are synthesized by a successive seeding-growth process using TSC as templating agent (Figures S2 and S3).29 The final volume fraction of Fe3O4 nanocrystals is 0.5%, and the concentration of Ag nanoplates is tunable from 1.25 to 20 mM. Because the Ag nanoplates and Fe3O4 nanocrystals are grafted with negatively charged citrate and acrylate groups on the colloid’s surface, the suspension is stable against aggregation when the magnetic field is applied. For all measurements of transmittance of the optical switch in this work, the incident light was set to vertically pass through the liquid film, while the orientation of magnetic field was manipulated to be perpendicular (H1) or parallel (H2) to the light path by a cylindrical magnet, and the transmitted signals are collected by the spectrometer on top of the glass cell (Figure 1). The tuning of transmittance was investigated by the in situ microscopic observation in dark-field mode and the corresponding transmission measurement (Figure 2). In the absence of a magnetic field, the Ag nanoplates dispersed in ferrofluid present various orientations due to Brownian movement of
Figure 1. (a) Scheme for tuning of transmittance of colloidal solution by changing the orientation of (c) Ag nanoplates in ferrofluid composed of (b) Fe3O4 nanocrystals. The scale bars in (b) and (c) are 50 and 500 nm, respectively. purchased from Norland. All chemicals were directly used as received without further treatment. Preparation of Aqueous Ferrofluid. Fe3O4 nanocrystals with average size of 10 nm were synthesized through a one-step hightemperature polyol process.27 In a typical synthesis, a mixture of PAA (4 mmol), FeCl3 (2 mmol), and DEG (15 mL) was heated to 220 °C in a nitrogen atmosphere with vigorous stirring. NaOH/DEG stock solution (4 mL, 2.5 mol/L) was then injected into the above solution, which was further heated for 30 min to yield Fe3O4 nanocrystals. After the reaction solution was cooled to room temperature, the nanocrystals were separated by centrifugation and washed three times with deionized water and ethanol. Finally, the highly charged magnetic nanoparticles (154 mg) were dispersed in deionized water (1.2 mL) to produce a stable aqueous ferrofluid with Fe3O4 volume fraction of 2.5%. Synthesis of Silver Nanoplates. Ag nanoplates were synthesized by a successive seeding-growth process. First, triangular Ag nanoplate seeds with average size of 35 nm were prepared by a wet chemistry method developed by Metraus and Mirkin.28 Typically, an aqueous solution of AgNO3 (0.1 mM, 25 mL), TSC (75 mM, 1.5 mL), PVP (17.5 mM, 0.3 mL), and H2O2 (90 μL) was mixed and stirred at room temperature, into which NaBH4 (100 mM, 250 μL) was rapidly injected to initiate the reduction. After 35 min of reaction, the solution gradually turned pale blue, which indicates the production of Ag nanoplates. The as-made Ag nanoplates were washed by deionized water three times and dispersed in 3 mL of water as the Ag seeds. To synthesize Ag nanoplates with larger size,29 Ag seeds (0.75 mL) were diluted with 10 mL of water and mixed with L-ascorbic acid (100 mM, 0.375 mL) and TSC (75 mM, 0.125 mL). A separate growing solution was prepared by mixing the solution of AgNO3 (1.0 mM, 20 mL) and TSC (1.5 mM, 0.125 mL). The growing solution was added dropwise into the seed solution through a syringe pump with an injection rate of 0.1 mL/min. After each 24 min, two-thirds of the solution was taken out and washed to produce Ag nanoplate of 300 nm, while the rest of the solution was kept for the following growth. Ag nanoplates with sizes of 0.4, 0.7, 1.0, 1.2, and 1.4 μm were obtained after the second, third, fourth, fifth, and sixth growing cycle, respectively. Measurement of Light Transmittance in Rotary Magnetic Field. Aqueous ferrofluid containing Ag nanoplates with designed concentrations was sealed between two parallel cover glasses, and placed in the light path of UV−vis spectrometer with glass perpendicular to the incident light. A NdFeB magnet or electromagnet was set up around the sample to provide an external magnetic field whose direction was parallel or perpendicular to the incident light. The
Figure 2. (a,b) Microscope images and (c) transmittance spectra for the suspension of Ag nanoplates with average size of 1.0 μm and concentration of 10 mM, when the magnetic field orientation is (a) perpendicular or (b) parallel to the light path. The scale bar is 50 μm, and the field strength is 280 mT. 13113
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colloidal particles. Once a magnetic field perpendicular to the incident light is applied to the optical switch, the Ag nanoplates show the complete or tilted triangular facets toward the light path, which blocks the incident light and decreases the intensity of transmission (low LT mode). When the field orientation is switched to be parallel to the incident light, all of the Ag nanoplates will show their edges along the light path, which minimizes the absorption, scattering, or reflection of Ag nanoplates and increases the transmittance accordingly (high LT mode). It should be noted that some Ag nanoplates with about 1 μm diameter were recorded as much larger dim brown disks in the microscope image because many of them are under or over focus, and the strong scattering and rapid vibration of Ag nanoplates also amplify their size in microscope images. The rotation of Ag nanoplates and switching between two modes were accomplished in several hundred milliseconds, which was recorded by a digital camera on microscope (see Movie 1). The transmission spectrum showing the average behavior of all Ag nanoplates in the system matches well with the microscopic observation. By setting the transmittance in high LT mode as the reference (100% LT), the transmitted signal decreased 20% in low LT mode. In the absence of magnetic field, the Ag nanoplates will vibrate in solution and present random orientations, so that its transmittance locates in the middle region between the above two. The mechanism of magnetic tuning of Ag nanoplates can be explained by the minimization of potential energy of “magnetic holes” in external magnetic field. According to the Skjeltorp theory, a nonmagnetic particle dispersed in the ferrofluid will behave like a magnetic hole, which has an equivalent but opposite magnetic moment to the total moment of the replaced ferrofluid when an external magnetic field is applied.22 For a symmetric spherical particle, its potential energy in ferrofluid when exposed to the external field has nothing to do with its orientation. However, for anisotropic particle in the same condition, the long axis tends to rotate toward the direction of external field, so that its magnetic moment will stay in the reverse direction of external field to minimize the total potential, which is similar to the rotation of magnetic compass. A recent work on the alignment of nanorod in ferrofluid reveals that the potential energy (U) of nanorod can be estimated by eq 1, where A is related to the dimension of long and short axis as well as the magnetic moment of nanorod and ferrofluid, H is the local field strength, and θ is the angle between external field direction and major axis of nanorod.23 2
2
U = AH sin θ
them are strong enough to overcome the enhanced Brownian motion caused by the elevation of temperature. Therefore, these nonmagnetic particles are theoretically responsible for the magnetic field above the freezing temperature and below the boiling temperature of water. In experiment, we have tested the magnetic response of Ag nanoplates in ferrofluid at about 10 and 70 °C, and tuning of transition is almost the same as that at room temperature. The applied magnetic field strength is significant to the rotation of Ag nanoplates as well as the sensitivity of the optical switch. Equation 1 not only shows the potential energy of anisotropic particle with specific tilting angle, but also tells the decrease of potential energy (ΔU) when the long axis of the particle rotates from any orientation to the direction parallel to the magnetic field. During this process, the potential decrease should be larger than the thermal energy of the nanoplates at room temperature, so that the anisotropic particle can overcome the Brownian motion and align in external field. Therefore, an adequate strong magnetic field is required to manipulate the Ag nanoplates. To determine the threshold value, the sensitivity of the optical switch, characterized by the light transmittance alteration (Δ(LT)) between the high LT and low LT mode, was measured when the magnetic field strength was gradually enhanced from 40 to 500 mT (Figure 3). The change of transmittance increases with the increase of
Figure 3. (a) Transmission alteration Δ(LT) for a typical suspension of Ag nanoplates with average size of 1.0 μm and concentration of 5 mM, when the solution is tuned by a rotary magnetic field with its strength rising from 40 to 500 mT. (b) The influence of field strength upon Δ(LT) at 800 nm when the magnetic field orientation is switched.
field strength, and nearly reaches its saturated value when the field strength rises to 280 mT. Further enhancing the magnetic field will no longer improve the device sensitivity, showing that all of the nanoplates have been aligned in this condition. Controlled by an appropriate magnetic field, the concentration of Ag nanoplates is tuned to optimize the sensitivity of the optical switch (Figure 4a,b). Here, Ag nanoplate with edge length of 1.0 μm was chosen as the research object in a magnetic field with specific strength (280 mT). As the Ag concentration gradually increases from 1.25 to 10 mM, the light transmittance alteration (Δ(LT)) increases accordingly, because the addition of Ag micro mirrors in solution will enhance the tuning effect of transmission in magnetic field. Further increasing the Ag concentration to 20 mM leads to a decrease of Δ(LT), probably because the increased ionic strength of solution will cause agglomeration of Ag nanoplates and decrease the amount of contributing Ag nanoplates for magnetic tuning. A 19% of transmittance alteration is much larger than the 2% of transmittance difference caused by the
(1)
The equation indicates the energy minimum occurs when the rod is aligned with the external field. On the basis of these discussions, both the alignment of nanoplates and the modulation of transmittance are easy to understand. It should be noted that the nanoplates can self-rotate around its major axis, so that not all of the nanoplates hold themselves exactly perpendicular to the incident light in low LT mode. Yet on average, they block the incident light and decrease the transmittance with no doubt. The tuning of transmission signal is effective in a large temperature range. It is well-known that the liquid crystal displays work normally above the melting temperature (Tm) of the material and below its clearing temperature (Tc), where the liquid crystal turns into an isotropic liquid. However, the colloidal Ag nanoplates and magnetic nanoparticles have no phase transition in water, and the magnetic forces exerted upon 13114
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Figure 5. Transmittance alteration Δ(LT) at 800 nm for a suspension of Ag nanoplates with size of 0.7 μm and concentration of 5 mM operated in a periodically orientation-switching magnetic field (280 mT).
be the most interesting topic when discussing the tuning of switching time. According to the magnetic moment of “magnetic holes”, the magnetic interactions between Ag nanoplates enhance with the increase of nanoplates size, leading to the shortening of switching time. However, as the nanoplates size increase, their mass increases, and the resistance exerted upon them by the fluid during rotation increases accordingly, which may prolong the switching time. Furthermore, the concentration of ferrofluid and Ag nanoplates as well as the strength and frequency of rotary magnetic field should also be considered to draw a conclusion. The theoretical simulation and deduction was beyond our capabilities. In experiment, it is found that the switching times for Ag nanoplates with different size are very close to each other. A faster measurement of transmittance in the level of microseconds may help to obtain the precise switching time. The use of Ag nanoplates extended the “nonmagnetic” assembly and tuning system further into the submicrometer scale and intrinsically improved the stability of the colloidal system. In the previous work,12 Al flakes with diameters of 1− 15 μm were applied to control the transmittance of the suspension, and a heavy liquid composed of toluene and diiodomethane was used to avoid the precipitation of Al flakes and maintain the response, so that the horizontally placed display unit can function for a long time. However, this solution does not work for display unit placed vertically, where all of the Al flakes will accumulate in the upper edge of the cell. The current work decreased the flake size into the submicrometer scale and utilized highly electrostatic repulsive Ag nanoplates and Fe3O4 nanoparticles to compose the suspension, which intrinsically enhance the stability of colloidal particles. In the mixing solution, the magnetite nanocrystals do not attach to the Ag nanoplates even after they have been exposed to magnetic field (Figure S4), making the nonmagnetic nanoplates typical “magnetic holes” with reverse magnetic moment in ferrofluid. Previous experience of colloidal Ag nanoplates dispersed in water indicates that precipitation will occur when their size reaches several hundred nanometers. Fortunately, in a thin glass cell, the Ag nanoplates can be agitated by the magnetic field more easily, which also guarantees the stable performance of the device. In a practical test, about 20 cycles of transmission switching in tunable magnetic field with an interval time of 3 s demonstrate the good reversibility of the magnetic optical switch in the working condition (Figure 5b). The slight inconsistence of transmission signals in parallel magnetic field (ΔT ≈ 0) is probably caused by the errors due to manual operation of permanent magnet.
Figure 4. Transmission alteration Δ(LT) at 800 nm for (a) suspension of Ag nanoplates with average size of 1.0 μm and increasing concentration from 1.25 to 20 mM, and (b) suspension of Ag nanoplates with fixed concentration of 5 mM and increasing size from 0.4 to 1.4 μm, when these solutions are manipulated by a rotary magnetic field of 280 mT.
change of magnetic field orientation for a pure ferrofluid, and it is comparable to that of device fabricated with magnetic nanoparticle attached aluminum flakes.12 In addition to the concentration, the size of Ag nanoplates is also critical to the sensitivity of the optical switch (Figure 4c,d). In all parallel experiments, the concentration of Ag nanoplates were fixed as 5 mM, and a magnetic field with specific strength (280 mT) was used to tune the transmittance. With the average edge length of Ag nanoplates increasing from 0.4 to 1.0 μm, Δ(LT) of corresponding samples increases accordingly, because the enlargement of nonmagnetic particle volume will enhance the magnetic response (including magnetic packing force and interactions) of these particles in ferrofluid and lead to better alignment in both two modes. When the Ag size is further increased beyond 1.0 μm, a gradual decrease of Δ(LT) is observed. In this stage, the precipitation of nanoplates and their attachment to the glass wall will take domination as compared to the magnetic alignment, so that Δ(LT) decreases due to the decreasing number of Ag micromirrors in solution. It should be noted that this unstable effect does not affect the steady tuning of transmittance when the Ag size is around several hundred nanometers. Furthermore, the vertical and horizontal magnetic field exerted in the working condition will agitate the nanoplates and disperse them back to the colloidal solution easily, although Ag nanoplates with several hundred nanometers may gradually precipitate in storage. Therefore, a stable switching performance is expected after the above optimization. The as-prepared optical switch has fast and reversible response to the tuning magnetic field. Figure 5a shows the time evolution of Δ(LT) when the perpendicular and parallel magnetic fields are manually switched with an interval of 1 s. The transmission signal changes synchronously as the external field is adjusted, and the transmission alteration (Δ(LT)) turns “on” and “off” in about 300 ms. The influence of Ag size might 13115
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The new tuning system based on the magnetic interaction of nonmagnetic submicrometer particles has some advantages that may help to greatly broaden the research and application of colloidal assembly. The “nonmagnetic” assembly system not only retains the superior characteristics of magnetic tuning, such as contactless, rapid, and reversible control, but also offers much more selectivity when choosing the materials, because there are numerous nonmagnetic colloidal particles with various shapes and sizes available to serve as the building blocks. Interesting applications may be achieved by combining the properties of these nonmagnetic materials with magnetic tuning strategy. For instance, this work utilized the magnetic tuning of Ag plate orientation to change the transmittance of the suspension, which could be a potential optical switch. Our recent work also demonstrated that the magnetic assembly of nonmagnetic polymer spheres can be used to fabricate photonic display unit. It is believed that in the near future, more and more applications based on the “nonmagnetic” assembly system will be explored and developed.
In summary, Ag nanoplates and Fe3O4 nanoparticle-based ferrofluids were utilized to fabricate a magnetically controlled optical switch with fast and reversible tunability in transmittance. The mechanism of transmittance tuning can be explained by the rotation of Ag nanoplate, whose long axis always follows the orientation of external magnetic field to minimize the potential energy. The sensitivity of the optical switch was optimized by choosing Ag nanoplates with appropriate size and concentration. The switching of transmittance responding to the magnetic field is fast and fully reversible. It is believed that magnetic tunable system based on nonmagnetic building blocks will soon find its application in novel functional devices.
ASSOCIATED CONTENT
* Supporting Information S
Morphology of magnetic nanoparticles and photos of ferrofluid with strong magnetic response; TEM images of Ag nanoplates with various sizes; and proofs of no attachment between Ag nanoplate and magnetic nanoparticles. Movie showing the rotation of Ag nanoplates and switching between two modes, accomplished in several hundred milliseconds. This material is available free of charge via the Internet at http://pubs.acs.org.
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4. CONCLUSIONS
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.Ge thanks the National Science Foundation of China (21001083), the Major State Basic Research Development Program of China (2011CB932404), the Shanghai Pujiang Program (10PJ1409800), SRF for ROCS (SEM), and the Fundamental Research Funds for the Central Universities for support. 13116
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the web on August 27, 2012 with an error in the linking for the Supporting Information files. The corrected version was reposted on September 11, 2012.
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