Microsegmented Flow-Through Synthesis of Silver Nanoprisms with

Article pubs.acs.org/JPCC

Microsegmented Flow-Through Synthesis of Silver Nanoprisms with Exact Tunable Optical Properties Andrea Knauer,*,† Andrea Csáki,‡ Frances Möller,† Carolin Hühn,† Wolfgang Fritzsche,‡ and J. Michael Köhler† †

Ilmenau University of Technology, Institute for Chemistry and Biotechnology, Dept. of Physical Chemistry and Micro Reaction Technology, Weimarer Str. 32, D-98693 Ilmenau, Thuringia, Germany ‡ Institute for Photonic Technologies, Dept. of NanoBiophotonics, Albert-Einstein-Str. 9, 07745 Jena, Thuringia, Germany ABSTRACT: In this work, a multistep microcontinuous flowthrough synthesis procedure for the generation of homogeneous, high-quality silver nanoprisms is presented. The particle synthesis is based on the wet chemical reduction of silver nitrate in the presence of the polyanionic polymer poly(sodium styrenesulphonate). To obtain a high yield of homogeneous prism-shaped Ag nanoparticles with a triangular base, two main experimental steps are necessary. The first step is the synthesis of seed particles. To match the quality criteria for small, homogeneous seed particles, the synthesis was carried out in a microcontinuous flow-through system. Constant residence times and an effective mixing of the reactants were realized by the application of the microsegmented flow technique. The advantage of good reactant mixing was also adapted in the second experimental step. The growth of silver nanoprisms by reduction of silver nitrate on the noncapped surfaces of the seed particles was again carried out within microfluid segments during a continuous flow-through synthesis. The obtained colloidal solutions of both, Ag seeds and Ag nanoprisms, were analyzed using differential centrifugal sedimentation, UV−vis spectrophotometry, and scanning electron microscopy. The size distributions of the product particles of the individual process steps were extremely narrow. For the Ag seed particles, an average particle diameter of 3.8 nm with a distribution half-width of 2.3 nm was found. The edge length of the Ag nanoprisms could be varied between 35 and 180 nm, while the size distribution remained narrow and the yield of particles of the desired shape high. Because of the strong sensitivity of the optical properties of the nanoprisms from the geometrical aspects, Ag nanoprisms promise a high potential for sensor applications. Constraints on nanoparticles presented by these applications, such as uniformity and narrow size distributions, can be met by microreaction technology. In particular, by applying a microsegmented flow, an improvement of the product quality can be achieved because of the enhanced segment-internal mixing and the suppression of a residence time distribution.

1. INTRODUCTION Silver nanoparticles attract great interest due their optical and electronic properties.1,2 On the one hand, Ag colloids show an intensive plasmon resonance with a sharp absorption band (localized surface plasmon resonance, LSPR), and on the other hand, silver nanoparticles are a very efficient material for nanoparticle enhancement of Raman signals.3,4 The resonance energy for the inelastic electromagnetic resonance is strongly affected by the shape5,6 and the size7,8 of the nanoparticles. Spherical silver nanoparticles show a strong plasmon absorption at the short-wavelength edge of the visible spectrum (around 400 nm), while the main dipole absorption wavelength of nonspherical particles can be shifted over the whole visible spectral range up to the near-infrared.5 Triangular prismatic, hexagonal prismatic,9 and disklike10 nanoparticles are particularly suited for a tuning of the plasmonic properties with nanoparticle size. The plasmon absorption of spherical nanoparticles splits into two or more components related to the main axes of the geometrical shape. In particular, the inplane oscillation causes strong long-wavelength absorption.11,12 © 2012 American Chemical Society

The application of noble metal nanoparticles for molecular or biochip labeling,13,14 in nanoelectronics15 or in information recording,16,17 is dependent on the availability of respective nanomaterials with a high homogeneity in particle shape and size. This aspect is a challenge for the preparation methods to supply the desired particles in a high yield and excellent quality by the help of a fast and simple synthesis method. In most cases, conventional synthesis protocols provide a mixture of nanoparticles of different shapes and sizes. A reliable and robust method for fabrication of monodisperse particles with high yield and suppression of undesired side-product particles remains difficult under conditions of conventional batch preparation techniques. However, the preparation of particles on-demand with a tunable size in a continuous synthesis process is highly desirable. Received: November 10, 2011 Revised: March 14, 2012 Published: March 15, 2012 9251

dx.doi.org/10.1021/jp210842g | J. Phys. Chem. C 2012, 116, 9251−9258

The Journal of Physical Chemistry C

Article

Figure 1. Schematic overview of the influence of different mixing conditions related to different preparation methods on the homogeneity of the surface charge distribution during the wet chemical synthesis of colloidal solutions and the consequences on the aggregation behavior.

finally to precipitation, which causes the breakdown of the colloidal state (coalescence). Therefore, a fast mixing of reactants and well-controlled transport and reaction conditions for all involved components are a central requirement for a high-quality nucleation and the formation of nanoparticle dispersions with high homogeneity and yield in regard to the desired product properties.20 These requirements were met by microreaction technology. This technique is able to realize very fast transfer of heat and matter21 and can be applied under constant and robust convective conditions. The problem of large residence time distributions, which is caused by the laminar flow in homogeneous fluids due to the formation of a parabolic velocity distribution, is overcome by the microsegmented flow technique.22,23 The microsegmented flow technique was successfully applied for the synthesis of different micro- and nanomaterials, for example, carbonates and other dielectric inorganic particles,24 polymer particles,25 semiconducting particles,26 and for noble metal nanoparticles.22,27,28 At higher flow rates, the transport-induced segment-internal convection leads to a very fast mixing of the reactants inside the fluid segments.29 This technique can be applied under constant fluid actuation and chemical conditions for the microcontinuous flow synthesis of nanomaterials,30 but can also be used for combinatorial experiments under variation of flow rates.31

The mechanism of nanoparticle formation is the main reason for obtaining larger distributions of particle sizes and the formation of side products. The nucleation as well as the particle growth is strongly dependent on the local concentration gradients of the reactants. These local concentration gradients are rapidly changing during mixing of the reactants. In particular, the nucleation is a time-critical process and affected by the mixing process. Additionally, the silver cations on the one side and the reducing agent, most of the ligands, and the anions on the other side are affecting the electrochemical potential of the forming particles in opposite directions. High local concentrations of silver ions and other interacting Lewis acids cause an enhancement of potential; high concentrations of the reducing agent and interacting Lewis bases cause a reduction of the particle potential. This is a critical point in terms of particle growth, because the attachment of silver ions at the particle surface and the conversion of the metal cations into metal atoms is an electrochemical process. In addition, the surface charge of the particle is crucial for the stability of the colloidal solution.18 High surface charges of the same polarity are responsible for an efficient electrostatic repulsion between the particles. Thus, a ζ potential of at least ±30 mV is necessary to suppress particle aggregation.19 A partial or a complete discharging of the particles leads to an increase of attractive interactions between the particles, to particle aggregation, and 9252



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In this work, the applicability of the microsegmented flow technique is investigated for the synthesis of triangular silver nanoprisms. The synthesis strategies for this kind of highly interesting novel material are manifold. In the literature, different batch methods and photomediated or photoinduced conversion methods for the growth of triangular silver nanoprisms are reported.32−37 The aim of this study is to demonstrate how the advantages of the microfluidic method can be used for the generation of high-quality prismatic particles and to show what promising results can be achieved by an adaption of the facile protocol for batch synthesis to the conditions of a microcontinuous flow process. To the best of our knowledge, this is the first report about the microfluidic synthesis of triangular silver nanoprisms in a micro flowthrough system under application of the microsegmented flow technique. The technique, demonstrated in this work, leads to an increasing yield in the desired shape with uniform particle size distributions, which causes, for example, a line width reduction of the absorption bands in the optical spectra, and it offers the possibility of an exact in situ tuning of the nanoprisms' physical properties.

The problem of colloid stability has to be addressed if the synthesis protocol should be transferred from a batch into a continuous-flow procedure (Figure 1c−e). At first, it is difficult to realize a droplet-wise addition into a continuous-flow process. Second, local concentration gradients during mixing must be smoothened as fast as possible. These problems cannot be overcome conveniently in a homogeneous-phase microfluidic process (Figure 1d). However, the application of the microsegmented flow promises a solution if the mixing process and nucleation can be realized so fast that particle aggregation can be neglected (Figure 1e). This can be achieved if high flow rates are applied under conditions of a homogeneous segmentation of the fluid and homogeneous dosing of the reactants. Best conditions for nanoprism formation are expected if both the seed formation and the particle growth are based on the fast segmented-internal mixing. Therefore, a five-step procedure was favored for realizing the microcontinuous-flow synthesis. 1st flow process: (i) Formation of microfluid segments by in situ mixing of sodium citrate solution with a premixed solution of sodium borohydride and sodium poly(sodium styrenesulphonate) (PSSS). (ii) Addition of silver nitrate solution by dosing into the preformed segments containing the other reactants. 2nd flow process: (iii) Formation of microfluid segments by a mixture of a colloidal solution of silver seeds and ascorbic acid. (iv) Addition of silver nitrate solution by dosing into the preformed segments containing the other reactants. subsequent batch process: (v) Completion of particle growth in the batch in the presence of a sodium citrate solution after collection of fluid segments and phase separation. 2.3. Experimental Setup and Fluidic Conditions. Both processes, the generation of the Ag seed particles as well as the Ag nanoprism growth, demanded an experimental setup, which was specially adapted to the requirements of the synthesis. The setup used for the Ag seed synthesis is schematically shown in Figure 2a. Five individual syringes were mounted on a multiaxis electronically controlled syringe pump system (Cetoni NeMESYS, Cetoni GmbH, Korbussen, Germany) with independently controllable axes. The syringes were connected via PTFE tubing with an inner diameter of 0.5 mm (Bohlender GmbH, Gruensfeld, Germany) to standard fluid connectors (IDEX Health & Science LCC, Oak Harbor, WA). To set the segmented flow, a carrier stream of an immiscible organic phase was presented first. The generation of segments took place at the first injection unit, where sodium citrate and, on the opposite side of the reactor channel, a mixture of PSSS and sodium borohydride were dosed into the stream. At a second injection unit, silver nitrate was added into the preformed segments. The respective flow rates and reactant concentrations are presented in Table 1. To achieve homogeneous, highquality particles and a narrow size distribution combined with a minimum average particle diameter, a high total flow rate was applied. The Ag+ reduction by borohydride is a rapid reaction, which leads to fast nucleation, and therefore, optimum mixing

2. EXPERIMENTAL SECTION 2.1. The Principle of Microsegmented Flow and Experimental Conditions for Nanoparticle Formation in Microfluid Segments. The batch protocols from Aherne et al.38 were transferred to the conditions of the microsegmented flow system. First, the literature-known protocols were miniaturized to milliliter scale. Both experimental steps, the synthesis of silver seeds and the growth of silver nanoprisms, were carried out in the conventional synthesis procedure as a reference, which is needed for a comparison to the results obtained by the microsegmented flow synthesis. The two different stages of product particles, the silver seeds as well as the Ag nanoprisms, were characterized by differential centrifugal sedimentation (DCS), UV−vis spectrophotometry, and SEM analysis. Subsequently, the successful batch protocols were recalculated according to the concentration and volume flows of the reactants for an implementation of the reaction into the microsegmented flow. For the generation of a segmented flow in a microchannel, the aqueous reactant solutions are dosed stepwise into a continuous stream of the carrier medium. Under stable fluidic conditions and homogeneous addition of the reactant solutions, uniform segments with equal segment sizes and intermediate distances are formed. 2.2. Synthesis Strategy for Silver Nanoprisms. The development of a microreaction process for the synthesis of silver nanoparticles was based on the batch protocol of Aherne et al.38 In this procedure, silver ions are reduced by sodium borohydride in the presence of the polyanionic effector poly(sodium styrenesulphonate) for formation of small silver seeds. These seeds are further enlarged by metal-catalyzed silver deposition from a silver salt solution by ascorbic acid. A cornerstone for application of this protocol for obtaining homogeneous silver nanoprisms is the careful dropwise addition of silver salt into a solution with an excess of reducing agent for the particle growth. Therefore, the concentration of silver ions is kept low during the whole process and a stronger increase of the particle potential by adsorption of silver ions is avoided (see scheme in Figure 1a). A fast merging of the solutions of reducing agent and silver salt can result in a charge compensation of nanoparticles, in particle aggregation, and in destruction of the colloidal state (Figure 1b). 9253



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Table 2. Distribution and Premixing of the Chemicals in the Syringes as Well as Used Concentrations and Microfluidic Conditions for the Ag Nanoprism Growth syringe

substance

1 2

carrier medium 5 mL H2O 0.075 mL C6H8O6 0.5 mL, 0.4 mL, 0.3 mL, 0.2 mL, 0.1 mL Ag seed NP AgNO3

3

Table 1. Distribution of the Chemicals in the Syringes, Applied Concentrations, and Flow Rates for the Microfluidical Synthesis of Ag Seed Particles substance

concentration

flow rate (μL/min)

1 2 3 4 5

carrier medium C6H5Na3O7 NaBH4 PSSS AgNO3

2.5 mM 10 mM 500 mg/L 2.5 mM

1000 474 14.2 23.6 94

flow rate (μL/min) 100 60

20

1, 0.4

40

seeds will lead to accordingly smaller product particles. To obtain Ag nanoprisms of different sizes, the amount of added Ag seed particles was varied. With the ability to alter the edge length of the Ag nanoprisms in a defined way, it is possible to tune the optical properties as well as the spectral position of the main plasmonic absorption band. 2.4. Product Characterization. The product particles were characterized using the techniques and instruments listed hereafter. The optical properties of the prepared Ag nanoparticles were characterized using UV−vis spectrophotometry (Specord 200, Analytik Jena AG, Jena, Thuringia, Germany). The SEM technique (Hitachi S-4800 FE-SEM, Hitachi High Technologies America, Inc., Schaumburg, IL) was utilized to analyze the shape and size of the obtained particles. The particle size distributionin the case of Ag nanoprisms, the Stokes equivalent sphere diameterwas determined by differential centrifugal sedimentation (DCS, DC 20000, CPS Instruments Inc., Newtown, PA). 2.5. Chemicals and Materials. All chemicals were used as received from the following suppliers. Sodium citrate (Merck KGaA, Darmstadt, Germany, purity: 99%), poly(sodium styrenesulphonate) (PSSS) (ACROS Organics, Morris Plains, NJ, MW: 70 000), sodium borohydride (Merck KGaA, Darmstadt, Germany, purity: 99%), and silver nitrate (Merck KGaA, Darmstadt, Germany, purity: 99%) were used for the synthesis of the Ag seed nanoparticles. For the nanoprism growth, ascorbic acid (Merck KGaA, Darmstadt, Germany, purity: 99.7%) is additionally required as a reducing agent. Perfluoromethyldecalin (F2 Chemicals Ltd., Lea, Lancashire, England) was used as a carrier medium. All solutions were prepared in ultrapure, particle-filtered water (filtration system: Aqua purification G 7795, Miele, Gütersloh, Germany) with a specific electric resistivity of 18.2 MΩ·cm. The experiments were carried out under clean-room conditions to keep the risk of cross-contamination low.

Figure 2. Experimental arrangements used for (a) the microfluidical preparation of Ag seed nanoparticles and (b) the growth of Ag nanoprisms.

syringe

concentration (mM)

3. RESULTS AND DISCUSSIONS 3.1. Microsegmented Flow Synthesis of Silver Seeds. The formation of silver seeds in a continuous-flow process was realized by application of a carrier flow rate of 1 mL/min and a total of the aqueous phase components of 606 μL/min in an experimental arrangement as shown in Figure 2a. These parameters correspond to an approximated dosing time of about 10 ms in the case of a 0.3 μL segment volume. Best conditions were found when the silver nitrate concentration was enhanced by a factor of 5 in comparison to the batch protocol. Obviously, a fast nucleation occurs. The microfluidic system seems to be more robust against potential enhancement by high Ag+ concentrations. The mixing is fast

conditions are required to keep the time interval of nucleation low. For the experimental step of the Ag nanoprism growth, three syringes, the respective syringe pumps, and two PEEK Tjunctions were required. The setup used for the Ag prism growth is shown in Figure 2b. A list of the required chemicals, the reactant concentrations, the premixing of the reactants, their distribution in the syringes, the employed volumes, and flow rates is given in Table 2. As was introduced by Aherne et al., the edge length of the Ag nanoprisms depends on the number of seed particles in the growth solution. At an equal concentration of metal salt, a low amount of seed particles leads to an extended edge length, whereas a larger amount of silver 9254



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The main differences in the homogeneity can be explained qualitatively by the assumption of a slower interdiffusion process of reactant solutions in the batch process (Figure 4, left). The slow concentration shift causes a broad nucleation interval. The first formed particles are already growing when further nuclei are formed. The significant portion of large particles in the case of the batch synthesis suggests a superposition of long-time nucleation and partial aggregation. The fast mixing in the case of the microsegmented flow process leads to a steep increase of nucleation rate in all volume parts of the formed segment (Figure 4, right). The started nucleation leads to a homogeneous decrease of silver ion concentration and consequently to a fast fall below the nucleation threshold. Therefore, the formation of further nuclei stops in the whole segment, and consequently, only a little but homogeneousgrowth of particles takes place. 3.2. Size Tuning of Nanoprisms in Segmented Flow Synthesis. The investigations on batch syntheses of Aherne and co-workers38 clearly showed the possibility of precise tuning the nanoparticle size by shifting the ratio of seeds and silver nitrate in the particle growth. This principle was also applied for the microcontinuous-flow synthesis. In the continuous-flow experiment, the seed concentration was varied by application of two input flows with varied flow rate: one containing only ascorbic acid, the other containing ascorbic acid with the same concentration and, in addition, the silver seeds. The high regularity of formation of silver nanoprisms of different sizes in relation to the seed concentration is well reflected by the centrifugal sedimentation spectra as well as by the optical spectra. The particle size (volume equivalent sedimentation diameter) was continuously enhanced from 21 over 24, 27, and 30 to 39 nm at 0.4 mM AgNO3, if the initial seed concentration is lowered (Figure 5a). The volume equivalent sphere diameters correspond to an edge length of 35, 60, 80, 130, and 180 nm. With variation of size, the longwavelength plasmonic absorption peak (in-plane dipole mode)

enough for initiation of a rapid nucleation, which is obviously connected with a rapid consumption of free silver ions. The formation of silver seeds seems to be related to a small time window and a comparatively homogeneous distribution of reactant concentrations. This results in a small band in the size spectrum of obtained particles (Figure 3). Ag seed particles,

Figure 3. Weight-based DCS size distribution spectra for batch and flow-through processed Ag seed nanoparticles.

which were synthesized in a micro flow-through process display an average particle diameter of 3.8 nm with a full width at halfmaximum (fwhm) of 2.3 nm. The data for average particle diameter and fwhm were obtained by weight-based DCS measurements. The half-width of silver seeds synthesized in the batch is significantly larger than the half-width obtained from the microcontinuous-flow synthesis. The mean particle diameter of the batch-synthesized Ag seeds is 4.2 nm with a full width at half-maximum of 7.5 nm.

Figure 4. Influence of the mixing conditions in batch and in microsegmented flow-through synthesis on the homogeneity of the obtained product particles (schematic illustration). 9255



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Figure 6. (a) Number-based DCS size distribution spectra of the Stokes equivalent sphere diameter of five different Ag nanoprism solutions. The particle dispersions were prepared in a micro flowthrough process using 1 mM silver nitrate solution. An estimate of the actual edge length, derived from SEM measurements, is given in the legend of the graphs. (b) Shift of the in-plane dipole mode in relation to the silver nitrate/seed particle ratio. The data can be fitted by a root function of the type λ = [((c/V) − b)/a]1/2 + d, in good agreement. The determined parameters are a = 3.1 × 10−5 mol·L−2, b = 0.8 nm2, and d = 508.4 nm for 0.4 mM AgNO3 and a = 6.6 × 10−5 mol·L−2, b = 1.8 nm2, and d = 505.7 nm in the case of 1.0 mM AgNO3. The layers show the normalized extinction spectra of the different colloidal solutions obtained from five different volumes of seed particles for (i) 0.4 mM AgNO3 and (ii) 1.0 mM AgNO3.

Figure 5. Number-based DCS size distribution spectra (a) of the sphere equivalent sedimentation diameter of microfluidically processed Ag nanoprisms and (b) the respective UV−vis spectra of the product solutions using 0.4 mM AgNO3. Because of the addition of five different volumes of Ag seed particle solution, Ag nanoprisms of different lateral sizes were obtained. An estimation of the typical edge length of each sample, derived from SEM analysis, is given in the legend in the diagram of the DCS size distribution spectra.

is shifted from 528 to 832 nm (Figure 5b). Here, the absolute extinction value is monotonously decreasing with increasing edge length, which is evident from the non-normalized optical spectra and thus a proof for the high quality of the product solutions and the reproducibility of the synthesis method. A completely analogous picture was obtained at a silver nitrate concentration of 1 mM. In this case, the particle diameter was increased from 24 over 26, 29, and 34 up to 45 nm (Figure 6a), which corresponds to an edge length of 40, 70, 90, 145, and 200 nm. The optical absorption is shifted from 564 to 858 nm (Figure 6b, gray line). The graphs in Figure 6b illustrate the relationship between the main dipole resonance wavelength and the ratio between silver nitrate concentration and volume of inserted Ag seed nanoparticles. The experimental data can be fitted very well by a root function of the type λ=

demonstrates the possibility for the precise adjustment of the optical properties of silver nanoprisms by simple variation of the microfluidic reaction parameters. The centrifugal sedimentation spectra (by particle number) show also the expected reduction of peak height with decreasing seed concentration (Figure 6). The exponent of the dependence of the measured particle number on the seed concentration is higher than 1. This indicates an increasing probability of prism formation with increasing seed concentration. The decrease of particle number found in the sedimentation spectra corresponds to the increasing size of the particles. The thickness of prismatic particles strongly corresponded with the diameter of the seeds. The lateral extension (area) results from the amount of available silver ions during the growth of silver nanoprisms. The size and homogeneity of particles was also confirmed by SEM imaging of selected samples. In Figure 7a−e, triangular

c /V − b +d a

The found root functions have a y-offset of about 500 nm, which correlates to the minimal main dipole resonance wavelength and thus to the minimal edge length. Figure 6b 9256



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Figure 8. Corresponding conventionally batch-processed Ag nanoprisms. The nanoprisms shown here were prepared under the same chemical conditions as those presented in Figure 7. The comparison of the respective counterparts shows the clear influence of the improved mixing conditions on the homogeneity according to the size and shape distribution in the case of microsegmented flow-through synthesis.

Figure 7. SEM analysis of the different product particles. The following volumes of Ag seed nanoparticles were used for the Ag nanoprism growth: (a) 500, (b) 400, (c) 300, (d) 200, and (e) 100 μL.

silver nanoprisms obtained by the microfluidic synthesis strategy are shown. For the purpose of direct comparison, in Figure 8a−e, the corresponding particles from the batch preparation method are shown. From this comparison can be clearly seen that the flow-through synthesis is enormously advantageous, especially for particles with a high aspect ratio. According to the homogeneous size and shape distribution, the presented flow-through obtained triangular nanoprisms are also able to withstand the comparison to nanoprisms from various other preparation methods, which were reported in the literature before.11,32,34,35,38

This is one benefit of the microsegmented flow-through synthesis. High mixing rates and low volumes lead to a suppression of local concentration gradients and thus to wellcontrollable reaction conditions, which are necessary for a fast nucleation and a regular particle growth. The microfluidic processing of triangular Ag nanoprisms is divided in two partial steps. In the first flow process, Ag seed nanoparticles were formed. The DCS analysis clearly shows a narrower fwhm and a lower average particle diameter in comparison with the batch proceeded Ag seeds. The microfluidically obtained Ag seeds had a 5.2 nm narrower fwhm, and the average particle diameter was shifted by 0.4 nm toward lower values. A high total flow rate of 1606 μL/min ensured optimal mixing conditions, with simultaneous reduction of risk of aggregation. Actually, no evidence of aggregation could be found in the DCS spectra of micro flow-through processed Ag seed particles. The second flow process leads to the generation of homogeneous Ag nanoprisms with an exactly adjustable edge length. The shape uniformity and the high yield of Ag nanoprisms with a triangular base area could be confirmed by SEM measurements.

4. CONCLUSIONS The synthesis of prismatic Ag nanoparticles having a triangular base area was successfully implemented in a microcontinuous flow-through system using the method of the microsegmented flow. Because of the fast segment-internal mixing, a clear improvement of the particle quality of both, Ag seed nanoparticles and Ag nanoprisms, was achieved. The timecritical conventional batch protocol could be transferred into the microsegmented flow-through synthesis without considering the initial time dependence of the addition of the metal salt. 9257



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(23) Theberge, A. B.; Courtois, F.; Schaerli, Y.; Fischlechner, M.; Abell, C.; Hollfelder, F.; Huck, W. T. S. Angew. Chem., Int. Ed. 2010, 49, 5846. (24) Frenz, L.; El Harrak, A.; Pauly, M.; Begin-Colin, S.; Griffiths, A. D.; Baret, J.-C. Angew. Chem., Int. Ed. 2008, 47, 6817. (25) Jai Il, P.; Saffari, A.; Kumar, S.; Guumlnther, A.; Kumacheva, E. Annu. Rev. Mater. Res. 2010, 40, 415−443. (26) Zhao, C. X.; He, L. Z.; Qiao, S. Z.; Middelberg, A. P. J Chem. Eng. Sci. 2011, 66, 1463. (27) Duraiswamy, S.; Khan, S. A. Small 2009, 5, 2828. (28) Aimable, A.; Jongen, N.; Testino, A.; Donnet, M.; Lemaitre, J.; Hofmann, H.; Bowen, P. Chem. Eng. Technol. 2011, 34, 344. (29) Malsch, D.; Kielpinski, M.; Merthan, R.; Albert, J.; Mayer, G.; Kohler, J. M.; Susse, H.; Stahl, M.; Henkel, T. Chem. Eng. J. 2008, 135, S166. (30) Wagner, J.; Kohler, J. M. Nano Lett. 2005, 5, 685. (31) Funfak, A.; Cao, J. L.; Knauer, A.; Martin, K.; Kohler, J. M. J. Environ. Monit. 2011, 13, 410. (32) Dong, X. Y.; Ji, X. H.; Jing, J.; Li, M. Y.; Li, J.; Yang, W. S. J. Phys. Chem. C 2010, 114, 2070. (33) Frank, A. J.; Cathcart, N.; Maly, K. E.; Kitaev, V. J. Chem. Educ. 2010, 87, 1098. (34) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (35) Murayama, H.; Hashimoto, N.; Tanaka, H. Chem. Phys. Lett. 2009, 482, 291. (36) Xue, C.; Metraux, G. S.; Millstone, J. E.; Mirkin, C. A. J. Am. Chem. Soc. 2008, 130, 8337. (37) Wu, X. M.; Redmond, P. L.; Liu, H. T.; Chen, Y. H.; Steigerwald, M.; Brus, L. J. Am. Chem. Soc. 2008, 130, 9500. (38) Aherne, D.; Ledwith, D. M.; Gara, M.; Kelly, J. M. Adv. Funct. Mater. 2008, 18, 2005.

The homogeneity of the size distributions is evidenced by DCS analysis for all individual edge lengths. The improvement of the particle quality leads to an improvement of the optical spectra. All individual samples show a narrow longitudinal dipole plasmon resonance peak, in which the spectral position varies between 530 and 860 nm depending on the particles' geometric dimensions. Thus, the plasmonic properties of the Ag nanoprisms become easily adjustable by the help of the microcontinuous flow-through method due to the facile adjustability of the particles' dimensions. These investigations show the high potential of the microsegmented flow-through method in fields of the wet chemical synthesis of nonspherical plasmonic noble metal nanoparticles.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0049 3677 69 3689. Fax: 0049 3677 69 3173 E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the funding from the DFG (KO 1403/22-1). Furthermore, we thank Steffen Schneider for the technical support. The authors also thank Alexander Gross, Mike Guenther, Adam Williamson, Thomas Schneider, and Matthias Thiele for the useful discussions.

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REFERENCES

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Microsegmented Flow-Through Synthesis of Silver Nanoprisms with

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