Silver Composite Particles Produced via Microfluidic

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Polyacrylamid/Silver Composite Particles Produced via Microfluidic Photopolymerization for Single Particle-Based SERS Microsensorics J. Michael Köhler,*,† Anne Mar̈ z,‡ Jürgen Popp,‡,§ Andrea Knauer,† Isabelle Kraus,⊥ Jaques Faerber,⊥ and Christophe Serra⊥ †

Institute for Micro and Nanotechnologies/Institute of Chemistry and Biotechnology, Technical University Ilmenau, Gustav-Kirchhof-Strasse 1, Ilmenau, Germany ‡ Institute of Physical Chemistry and Abbe School of Photonic, Friedrich-Schiller-University Jena, Helmholtzweg 4, Jena, Germany § Institute of Photonic Technology Jena, Albert-Einstein-Strasse 9, Jena, Germany ⊥ University of Strasbourg, 4 rue Blaise Pascal − CS 90032 − F-67081 Strasbourg cedex, France ABSTRACT: A micro-continuous-flow process was applied for the preparation of swellable polyacrylamide particles incorporating silver nanoparticles. These sensor particles are formed from a mixture of a colloidal solution of silver nanoparticles and monomer by a droplet-based procedure with in situ photoinitiation of polymerization and a subsequent silver enforcement in batch. The obtained polymer composite particles show a strong SERS effect. Characteristic Raman signals of aqueous solutions of adenine could be detected down to 0.1 μM by the use of single sensor particles. The chosen example demonstrates that the composite particles are suitable for quantitative microanalytical procedures with a high dynamic range (3 orders of magnitude for adenine).

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The introduction of metal particles in polymer particles is of particular interest due to the complementary nature of the physical and chemical properties of both material types. A polymer matrix can be elastic, transparent to light, dielectric, and swellable. Furthermore, it can form a hydrogel and a nanoporous matrix. Additionally, the polymer matrix can be permeable for diffusion of molecules whereby selectivity in the permeability is realized by modifying the molecular functionalization. The response to mechanical, chemical, and thermal stress as well as swellability and porosity can be controlled by the cross-linking of the polymer chains. In contrast, noble metals, e.g., silver, are characterized by a high density, a high efficiency for light absorbance, high electron mobility, and low permeability. However, they also show a high chemical activity at their surface. A combination of both material types allows a tuning of physical and chemical properties over a wide range, depending on the component ratio and the local distribution. The inclusion of metal nanoparticles in microfluidically prepared polymer microparticles is carried out either via in situ synthesis of metal nanoparticles within the process of polymer particle formation or by incorporation of preformed metal nanoparticles inside the polymer matrix in the case of silver nanoparticles inside a photopolymerized particle.13 Both techniques supply a rather homogeneous distribution of the

olymer composites and in particular polymer/metal composite materials feature special mechanical, electronic optical, and chemical properties, which has led to a growing interest for their application in many fields.1−5 The production of composite materials is either carried out by the mixing of components or via in situ formation through chemical synthesis.6 The characterization of these materials is defined by the ratio of components and their spatial distribution regarding the size and shape of the internal interfaces, as the internal geometry is of major importance. The transport of charge, light, elasticity, and permeability as well as catalytic properties and the interaction with biological cells is dependent on the internal geometry. Local homogeneity in these properties can only be expected if a homogeneous distribution of one component in the other is achieved. Often, regular and small components are required to combine homogeneity with a high internal interface area and short distances between the single domains. Microparticles represent a suitable form for handling and application of a composite material. They can be produced with high homogeneity in size and chemical composition. A particularly narrow distribution of particle sizes can be achieved by application of a droplet-based microfluidic technique.7,8 Within a continuous flow process, primary formed droplets of monomers are polymerized by a photoinitiated radical reaction.9 Such microfluidic techniques can also be applied to generate regular composed microparticles as core/shell particles and so-called Janus particles.10−12 © 2012 American Chemical Society

Received: September 28, 2012 Accepted: December 3, 2012 Published: December 3, 2012 313

dx.doi.org/10.1021/ac302751t | Anal. Chem. 2013, 85, 313−318

Analytical Chemistry

Article

main emission at 366 nm). A residence time of less than 1 s in the irradiation zone was sufficient for initiation of the polymerization. The polymer particles were collected under PDMS and then washed five times with heptane and five times with ethanol. The particles were dried in air under ambient conditions or stored under ethanol. The silver enforcement was realized in batch. A scheme of the whole preparation process is shown in Figure 1. The process chain involves the two-step micro-

silver nanoparticles in the polymer matrix. However, the achieved silver content is rather low, as both the introduction of a high mass percentage of preformed metal nanoparticles and the application of a very high concentration of silver ions or compounds during an in situ formation is difficult. An alternative is the enhancement of silver content by an additional silver deposition after the microparticle formation. This principle is well-known from the silver-catalyzed silver deposition by reduction of silver ions and allows the realization of high silver contents. Therefore, a strategy was developed to enhance the silver content of microfluidically prepared polymer/silver composite microparticles. Swellable polymer composite particles with a high content of silver nanoparticles are of interest for analytical measurements using surface-enhanced Raman scattering (SERS). This technique allows an increase in the sensitivity of vibration spectrometry by several orders of magnitude.14−18 The use of microparticles for sensing in biotechnical microsystems19 and the combination of a highly efficient SERS material with particle techniques is of particular interest for sensing in automated screenings and in a microfluidic environment.20,21 Here, the synthesis of this type of composite particles was investigated and the applicability for single-particle-based sensing via SERS is demonstrated.

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EXPERIMENTAL SECTION The technique of microsegmented flow was used for the preparation of silver nanoparticles with very narrow distribution of size and shape. This technique allows the formation of regular small silver nanoparticles and gold/silver core/shell particles.22 Furthermore, the generation of prismatic silver nanoparticles in high yield can be realized by applying this preparation method. The size of the particles can be tuned by the ratio of applied metal seeds and dissolved reactants.23 A high quality of metal nanoparticles is achieved by a fast reactant mixing. The mixing is based on a regular and efficient flowinduced internal convection inside fluid segments and a very reproducible transport of segments through the microcapillaries. Silver seeds are synthesized by reducing silver nitrate in aqueous solution via sodium borohydride. This is carried out in the presence of the sodium salt of polystyrenesulfonic acid, mixing the components in microfluidic segments, which are embedded in a stream of a perfluorinated alkane. These particles can be enlarged regularly by a silver-catalyzed silver deposition using ascorbic acid at room temperature whereby nanoprisms are formed.24 The transfer of this method into a microfluidic process using fast moving fluid segments leads to a significant improvement in the homogeneity of nanoparticles23 and was, therefore, applied here. The composite particles are formed by a premixing of inorganic and organic components followed by regular droplet formation and the photopolymerization process. Polydimethylsiloxane (PDMS, 500 cSt) was applied for the viscous carrier fluid to realize a laminar flow in a micro-coflow arrangement for the droplet generation.25 Syringe pumps (Harvard) have been used to get a continuous fluid actuation with low pulsation. The monomer mixture containing all reactants was pressed through the opening of glass capillaries inside the coflow device to form a liquid jet. The droplets are generated regularly by the spontaneous decay of this jet into single droplets. The photopolymerization was realized by irradiation of the moving droplet inside the FEP capillary with UV light (Mercury source,

Figure 1. Process strategy and principle of microfluidic arrangement.

continuous-flow synthesis of silver nanoparticles using the microsegmented flow technique (Figure 1a), the synthesis of polymer composite particles in a coflow device (Figure 1b), and the silver enforcement (Figure 1c). The formation of monomer droplets in the microfluidic device was monitored by a microscope video system. The droplet formation inside the streaming carrier liquid is illustrated by microscopic video shots (Figure 2). Triangular silver nanoprisms were obtained in high yield and high size homogeneity by the segmented-flow method (Figure 3). The silver nanoparticles preformed in aqueous solutions were mixed with acrylamide (monomer). Therefore, acrylamide was

Figure 2. Formation of droplets of the monomer mixture in the microfluidic device before UV-irradiation for initiation of polymerization. 314



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droplets within less than about 10 s. The formed particles were collected in a glass vessel. The size of the droplets can be controlled by the diameter of the glass capillary and by the flow rates. In the applied microfluidic arrangement, droplet sizes between about 0.7 mm and about 50 μm have been realized. Particles with a comparatively narrow size distribution are synthesized by this microfluidic method. After drying, composite particles of about 0.3 mm diameter were obtained from droplets of about 0.45 mm original diameter (Figure 4).

Figure 3. TEM images of triangular silver nanoprisms obtained by micro-continuous-flow synthesis using the segmented-flow method.

Figure 4. Polymer/Ag composite particles: (a) optical image of a group of particles; (b) SEM image of the particle surface.

dissolved in a colloidal solution of silver nanoparticles and mixed with a cross-linker (bisacrylate) and a photoinitiator. As a result, a colloidal aqueous solution of silver nanoparticles containing all components for microparticle formation was obtained. This solution was guided through the glass capillary in the micro-coflow device as described above. The SERS measurements were carried out in a conventional micro-Raman setup (LabRam, Horiba Jobin-Yvon). The spectrometer equipped with a 300 lines/mm grating was combined with an inverse Olympus microscope. The frequency-doubled Nd:YAG laser (excitation wavelength 532 nm) was used as the laser source. The laser was focused onto a microbead within a microcuvette with the help of an Olympus 50× microscope objective. The incident laser power present on the sample was approximately 6 mW. The 180° backscattered light was detected with a back-illuminated CCD camera (1024 × 512 pixels). For the SERS measurements, adenine of analytical grade was purchased from Sigma-Aldrich. The dilutions were prepared using distilled water.

This size change corresponds to a volume shrinkage of about 70%. The particles were blue-green colored due to the included plasmonic silver nanoparticles. They appear opaque (Figure 4a) because of their porous structure. The observed color, which corresponds to the known plasmon absorption of the applied silver nanoparticles, is evidence for the incorporation of these nanoparticles at high density in the volume phase of the composite particles. The roughness of the particle surface was in the submicrometer range (Figure 4b). The submicrometer pores are probably the largest openings in a hierarchy of structures in the pore network including pores down to the lower nanometer level. A decrease in PDMS flow rate to 100 μL/min resulted in a final particle size of about 0.35 mm after drying. The larger particles were washed with heptane and ethanol and then treated with a freshly prepared mixture of 0.1 M ascorbic acid and 10 mM silver nitrate in aqueous solution. The deposition of silver on the surface of the particles could be observed within a few seconds as a shift in color from bright green-blue to black. Metallic silver was not formed in the solution but only on the particle surface. This fact suggests a deposition mechanism catalyzed by the particle surface. The deposition starts immediately after mixing and demonstrates a crucial role of the included silver nanoparticles in the catalyzed silver deposition. In contrast, an increase in temperature was required for a fast silver deposition on the polymer microparticles without any pre-existing metal nanoparticles. After being washed and dried, the particles were characterized by SEM. The particles show a rather irregular deposition of silver (Figure 5) and in many cases a deformed surface reflecting the shrinkage of the particles during drying. The surface of the particles was more or less dense, covered by silver particles with sizes in the submicrometer range and by particle aggregates with typical sizes of a few micrometers (Figure 5). A more homogeneous distribution of silver particles was achieved by the silver deposition on particles in a mixed solution (86% water/14% ethanol) containing 30 mM ascorbic acid and 3 mM silver nitrate (Figure 6). The sizes of metal nanoparticles obtained after 10 min (room temperature) are in the upper nanometer and lower submicrometer range. Larger aggregates were not observed. It is supposed that silver was not

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RESULTS AND DISCUSSION A micro-coflow arrangement consisting of an outer glass tube and a glass capillary was used for the synthesis of the primary composite particles. Therefore, the polyacrylamide and the photoinitiator were dissolved in the colloidal solution of silver nanoprisms. The solution remained blue. This color was caused by the long-wavelength plasmonic absorption of the triangular silver nanoparticles inside the colloidal solution. The nanoparticle-containing monomer mixture was pumped through a glass capillary with a final opening of about 0.25 mm internal diameter. The glass capillary was fixed in the central part of the outer glass tube. Droplets of the polymer mixture with a diameter of about 0.45 mm were formed by injecting the monomer mixture into an embedding stream of PDMS as shown in Figure 1b. A PDMS flow rate of 150 μL/min and a monomer flow rate of 20 μL/min have been applied for the droplet generation. After formation in the glass tube, the droplets of the reaction mixture were conducted through a FEP tube of 0.5 mm internal diameter. In a tube section of about 10 mm length, the droplets were exposed to the UV radiation for an exposure time of about 0.7 s. A mercury high pressure source was used to supply the UV light. The light source was connected to the FEP tube by a quartz fiber cable. The photoinitiated polymerization leads to a solidification of the 315



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Figure 7. SERS effect on the composite sensor particle: comparison of Raman signals of adenine solution (0.1 mM), of the pure composite particle, and of the composite particle in contact with the adenine solution.

Figure 5. Ag-enforced polymer/Ag composite particles. Silver enforcement after drying of polymerized particles: particles showing irregular shrinking structures and nonhomogeneous silver deposition on the surface.

solution. The pure composite particle shows no significant fingerprint information. The features appearing in the spectrum of the pure composite particles are caused by the polymer matrix. The broad bands between 1000 and 1600 cm−1 can be explained by the burning effects of the polymer matrix. These burning effects can be caused by the enhanced electromagnetic field at the enclosed silver particles affecting the matrix. In contrast, sharp and intensive Raman peaks were found if the adenine solution came into contact with the sensor particle. Namely, the peaks at 738 cm−1 (ring breathing) and at about 1336 cm−1 (ring stretching) are very strong and characteristic for the analyte molecule.26 This spectrum proves the high efficiency of the SERS effect from the applied sensor particle. Furthermore, it was demonstrated that the SERS peak intensity correlates strongly with the adenine concentration. SERS signals of adenine can be observed down to a concentration of 0.1 μM (Figure 8a). The change within the fingerprint region for a concentration of 1 × 10−7 M compared to the other concentrations is due to the fact that the background signal of the microbead itself obscures the analyte information. However, the plot of the signal intensity at 738 cm−1 shows that single-particle-based SERS sensing is suitable

only deposited at the surface but also in the inner part of the particles. Obviously, the slower silver growth rate in the slightly diluted solution and the ethanol content promote a more regular deposition of the silver particles. The high porosity of the polymer matrix is a precondition for an efficient deposition of silver on the primarily included silver nanoparticles inside the polymer matrix. So, silver ions as well as the molecules of reducing agent can diffuse into the inner part of the composite particles and undergo the metal-catalyzed silver deposition, leading to the incorporation of a larger amount of distributed metallic silver inside the composite particles which was intended to enhance the SERS sensitivity. The SERS activity of single-polymer composite sensor particles after silver enforcement was tested. Therefore, first a Raman spectrum of the pure composite particle was measured as shown in Figure 7. Subsequently, the polymer composite sensor particle was covered with test solution (0.1 mM adenine). A Raman spectrum of the pure solution and a spectrum of the sensor particle in contact with the adenine solution were recorded (Figure 7). The effect of the sensor particle is well reflected by the comparison of the three spectra (Figure 7). No Raman signal was observed in the pure adenine

Figure 6. Particle surface after Ag enforcement (without intermediate drying): (a) SEM image of larger surface area part (lower magnification); (b) SEM image of a surface region demonstrating a certain variation in the size of silver particles after silver enforcement. 316



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Figure 8. Sensitivity of single-particle Raman sensing: (a) spectra obtained for different solution concentrations of adenine; (b) Raman scattering intensity at 738 cm−1 depending on adenine concentration.

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for quantitative measurements of adenine concentrations over several orders of magnitude.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

CONCLUSION

Notes

The authors declare no competing financial interest.

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The investigations show that microfluidic techniques can be applied advantageously for the preparation of polymer composite sensor particles for optical readout. Polyacrylamide particles of homogeneous size containing silver nanoparticles can easily be synthesized by a continuous flow process in which a mixture of acrylamide, a cross-linker, and a photoinitiator with a colloidal metal nanoparticle solution is released in microdroplets by means of a coflow device and polymerized by UV initiation. These primary formed composite particles are transferred to highly SERS-active sensor particles by a chemical silver enforcement, leading to nanoporous microparticles with a high content of plasmonic silver. The particles swell in the analyte solution, and it is assumed that the analyte molecules diffuse efficiently through the polymer network and come into direct contact with the surfaces of the silver nanoparticles. As a result, a strong SERS signal is achieved. The application of micro-continuous-flow processes for both the synthesis of seed nanoparticles and the composite particles leads to a high homogeneity of particle sizes at the nanoscale (seeds) as well as at the microscale (polymer particles). This probably ensures the best preconditions for achieving highly homogeneous distributed metallic silver inside the polymer matrix after the chemical silver enforcement, resulting in sensor particles with strong SERS activity. The microfluidic synthesis of the composite particles leads to a high reproducibility in the transport processes during silver enforcement as well as in the analytical application during the SERS measurements. The high analytical potential of the synthesized composite particles is due to the combination of the microfluidic methods and the application of the photochemical formation of a nanoporous material containing SERS-active silver particles in high concentration. The applicability of these single-polymer composite particles for molecular sensing was demonstrated for aqueous solutions of adenine. This example proves that this single-particle SERS method (“SP-SERS”) is suitable for the quantification of concentrations down to the submicromolar range.

ACKNOWLEDGMENTS M.K. is grateful for a research stay and a guest professorship at the University of Strasbourg, which allowed him the opportunity to conduct the microfluidic particle synthesis experiments. The investigations of the microfluidic preparation of silver nanoparticles were supported by the DFG (KO1403/ 22-1).

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Silver Composite Particles Produced via Microfluidic

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