Optofluidics technology based on colloids and their assemblies
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- Lee, S., Kim, S., Kang, J. et al. Microfluid Nanofluid (2008) 4: 129. doi:10.1007/s10404-007-0218-8
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Optofluidic technology is believed to provide a breakthrough for the currently underlying problems in microfluidics and photonics/optics by complementary integration of fluidics and photonics. The key aspect of the optofluidics technology is based on the use of fluidics for tuning the optical properties and addressing various functional materials inside of microfluidic channels which have build-in photonic structures. Through the optofluidic integrations, fluidics enhances the controllability and tunability of optical systems. In particular, colloidal dispersion gives novel properties such as photonic band-gaps and enhanced Raman spectrum that conventional optofluidic devices cannot exhibit. In this paper, the state of the art of the colloidal dispersions is reviewed especially for optofluidic applications. From isolated singlet colloidal particles to colloidal clusters, their self-organized assemblies lead to optical manipulation of the photonic/optical properties and responses. Finally, we will discuss the prospects of the integrated optofluidics technology based on colloidal systems.
KeywordsOptofluidicsColloidal crystalsColloidal clustersNanosphere lithographySurface enhanced Raman scattering
Recent advances in microfluidics and nanophotonics have led to the newly emerging field known as optofluidics. Optofluidics is the complementary merging of photonics and fluidics. In optofluidic systems, fluids give fine adaptability, mobility and accessibility to nanoscale photonic devices which otherwise could not be realized using conventional devices (Psaltis et al. 2006; Monat et al. 2007a, b). However, homogeneous fluids have regulated properties with limited functionalities. Fluids should have unique and novel properties in order to develop optofluidic systems which are applicable for a wide range of practical problems. To this end, various colloidal particles are added into the fluids to provide specific physical, chemical and biomolecular functionalities. This introduction of colloidal dispersion will lead to a breakthrough for optofluidic systems.
Colloidal systems have been extensively studied since Thomas Graham (1805–1869) first used the terminology of “colloids” in his 1861 paper entitled “Liquid Diffusion Applied to Analysis”. In particular, recent developments in the field of molecular dynamics and photonics have been driving another dramatic evolution in colloidal science.
Once, colloids were regarded as another physical state of materials, because they have unique properties in terms of thermodynamics, electrostatics, optics and rheology. Now, they are usually defined as dispersions of particles, emulsions, supramolecules and molecular assemblies ranging from 1 nm ∼10 μm in size, and thus far, many of their properties have been uncovered. From a materials point of view, colloidal systems cover nearly all materials, including organics, ceramics, metals, and semiconducting materials. Different colloidal systems show different properties depending on the materials of the system (Russel et al. 1989).
Colloidal systems exhibit a wide range of physical and chemical properties. Colloidal science is not only related to classical physics but also to areas of modern physics such as the quantum mechanics, nano-plasmonics, and photonics (Ozin et al. 2005). Specifically, 2D colloidal arrays and nanoparticles made of noble metals interact strongly with light in a process known as surface plasmon resonance (SPR) and can localize light. These advanced properties are applicable to nanophotonic devices such as SPR waveguides, surface enhanced Raman scattering (SERS) active materials and colloidal lasers with ultra-high Q factors (Prodan et al. 2003; Levin et al. 2006; Knight et al. 1995; Vernooy et al. 1998).
Surface chemistry plays an important role in colloidal chemistry. Given that colloidal systems have large surface areas, chemical modification of their surface charges, functional groups or hydrophobicity lead to different phase behavior. In particular, colloidal interactions impose non-hydrodynamic forces and affect fluid dynamics, as colloidal materials are dispersed in flowing mobile liquids (Graf et al. 2003). Dye doping and labeling have led to the applications of nano-bar-coding for biomolecular detection purposes and optical gain for photonic devices (Battersby et al. 2000). Chemical preparation of metal shells on the colloidal particles enables high-efficiency SPR and SERS materials for highly sensitive media (Wang et al. 2007). In addition, the physical and chemical characteristics of colloidal dispersions are closely related to photonics and fluidics systems. Colloidal particles can be modulated under optical or fluidic phenomena. On the other hand, photonic responses and fluidic behaviors can change according to the colloidal system.
Colloidal materials can also be organized into nanoscale structures. From small colloidal clusters to 2D and 3D bulk colloidal crystals, the capillary force, electrostatic force and specific binding of biomolecules guide the evaporation-induced self-assembly of colloidal building blocks as well as the self-organization caused by thermal and mechanical convection, electrophoresis and sedimentation (Caruso 2004). Colloidal clusters composed of a small number of all identical spheres are known as colloidal molecules, which can be used as building blocks for novel colloidal crystal structures just like atoms or molecules form crystalline materials. These colloidal assemblies have a completely different range of behavior compared to isolated colloidal particles in dispersion. Colloidal molecules have different light scattering properties and resonance modes known as whispering gallery modes (WGM) (Knight et al. 1995; Vernooy et al. 1998). Colloidal crystals have special features as metamaterials and photonic crystals. Colloidal crystals exhibit unique light dispersion phenomena and possess the forbidden energy levels of photons (i.e., photonic band gaps).
In this review, optofluidic systems integrated with colloidal assemblies are discussed in terms of their structures and functionalities. The characteristics of colloidal dispersions and their chemical modification methods are included for applications related to photonics and optofluidics. Also, we outline recent developments in the optical modulation of colloidal particles and the flows driven by colloids in microfluidic systems. Finally, recent progresses in optofluidic integration of colloidal photonic units with microfluidic devices are presented in detail.
2 Physical and chemical aspects of colloidal particles for optofluidic devices
2.1 Control of photonic properties by colloids
Recently, synthesis processes for colloidal dispersions have been expanded from dielectric materials to metals and semi-conductors. In addition, core-shell particles with multi-layer coatings can be synthesized via chemical processes (Graf et al. 2002; Caruso 2001; Pham et al. 2002; Jackson et al. 2001). When metal nanostructures are located with nanoscopic gaps among them, hot spots can exist that result from strong localization of the electromagnetic field. Core-shell particles with dielectric cores and metal shells are one of these nanogap structures, and such colloidal nanoshell particles exhibit strong SPR and SERS activity (Sun et al. 2002; Jackson et al. 2003; Malinsky et al. 2001). This originates from the interaction of electromagnetic waves and oscillating surface charges generated on the inner and outer surfaces of the metal shells (Prodan et al. 2003). In particular, the thickness of gold and silver shells onto colloidal particles can be controlled to nanometer scale accuracy. According to the charge distributions, plasmon resonance has different plasmon energy states of bonding plasmons with low energy and anti-bonding plasmons with relatively higher energy. This is analogous to the molecular orbital theory of quantum mechanics.
In biotechnology, fluorescence detection continues to be an important subject as the techniques associated with it have been used in many areas, including biosensors, and nano-bar-coding and labeling of DNA biomolecules and cells. In general, high sensitive photon detectors are required to pick up the weak signals of fluorescent dye. In addition to use with high power light sources, high sensitive detectors such as those used in nitrogen-cooled CCD cameras or photomultiplying tubes (PMT) have been introduced to a fluorescence measurement setup to increase the detection range. Occasionally, signal processing is needed as lock-in-amplifiers are used to integrate the collected photon signals and finally to increase the S/N ratio. However, all of these units are expensive, complicated and bulky.
2.2 Modulation of colloidal suspensions by optics
Inside of the microfluidic channels, the fluid flow is dominated by viscous force rather than by inertial force. In this case, Reynolds number is very small and flow becomes laminar. Considering the nature of a laminar flow, mixing or sorting objects that are dispersed in a laminar flow is extremely difficult and has raised very challenging issues in the field of microfluidics. In particular, optical modulation of such systems has been suggested over the past 20 years.
Also, optical tweezers were used to induce the angular momentum from the trapped objects. When circular polarized light is used to trap birefringence microparticles, the transferred angular momentum spins the particle up to several hundred Hertz. Recently, the optical manipulation of fluids was performed using optical tweezers. Programmable holographic optical tweezers were used to induce fluid streams inside of microfluidic channels from a single light source. Terray et al. investigated optofluidic pumps and valves using time-varying optical traps controlled by scanning mirrors (2002). Ladavac and Grier reported that the holographic optical tweezers could be used to transfer the angular momentum of orbital into trapped particle rings (2004). As a result, colloidal particles migrated through the liquid mediator. Neale et al. fabricated birefringence microgears for optofluidic micropumps with spinning characteristics (2005).
2.3 Control of fluids by colloidal system
It is well known that the fluidic property of a solution is modulated by the dispersing colloidal particles. The rheological properties and stability of the colloidal dispersion are affected by the hydrodynamics, Brownian motions, electrostatics, dispersion forces and depletion forces. Typically, the surface and interfacial tension is reduced as the concentration increases while the viscosity increases. Given that viscosity is a function of the temperature and time-scale, shear thinning or thickening occurs with variations in the shear stress (Russel et al. 1989). Recently, much attention has been paid to the control of fluid motions by the optofluidic actuation of dispersing colloidal particles. Inside of microfluidic chips, the optofluidic properties of colloidal particles enable easy actuation of the fluid motion. Fine control of infinitesimal amounts of fluid samples can be modulated by an applied light source.
Recently, the micromanipulation of fluids has received an enormous amount of attention due to the increasing demand for micro-total-analysis-systems (micro-TAS) based on microfluidic platforms. However, complicated micropumps and valves, surface chemistry, electrode designs and various substrates are continually needed to control the biomolecules and living cells in the microflows. In a recent paper, Liu et al. reported that a colloidal dispersion with photothermal effects could be used to move fluids with a controllable speed and direction using light (2006). Photothermal nanoparticles (PNP) transform the photon energy into the thermal energy, and the thermal energy drives the dispersion with light of sub-mW power. This phenomenon is different from Marnagoni’s effect, which originates form the gradient of the surface tension.
3 Self-assembled colloidal structures for optofluidics applications
3.1 Colloidal photonic molecules
Spherical confinement generates colloidal crystals in spherical shape, unlike conventional colloidal crystals formed on flat plane substrates. For a length scale smaller than the capillary length, droplets maintain a perfect spherical shape and offer confinement during crystallization. Unlike microfluidic channels and cylindrical capillaries, a liquid droplet has a deformable and mobile interface which permits the migration of materials. If the droplet contains colloidal particles, they are aggregated as the size of the droplets become smaller during the removal of solvent. When the number of particles is smaller than several tens, they form colloidal clusters (Cho et al. 2005a, b; Manoharan et al. 2003; Yi et al. 2004). For much larger number of particles, spherical colloidal crystals are usually formed (Yi et al. 2002; Moon et al. 2004a, b; Kim et al. 2006; Velev et al. 2000). A microfluidic device is good tool for the generation of monodisperse droplets with controlled sizes and shapes. Yi et al. succeeded in a continuous preparation of particle-containing monodisperse droplets using microfluidic devices (2003a). Two immiscible fluids, an aqueous suspension and silicone oil, were introduced in the microfluidic device, and the continuous oil phase pinched off the aqueous suspension at a T-junction. As the silicone oil absorbed the water from the aqueous phase, the droplet size was gradually reduced as it moved along the channels. After the complete removal of the water, colloidal clusters were generated in the oil phase.
The particles always assemble into clusters with the same geometry if the number of contained particles in the emulsion droplets is uniform. Although a microfluidic device generates highly monodisperse droplets, the number of the particles contained in the emulsion droplets follows Poisson’s distribution and produces clusters of different configurations depending only on the number of the constituting particles (Yi et al. 2003b). However, the geometry of the clusters fabricated by microfluidic devices is much more regular compared to the use of other methods such as emulsification by homogenizer and shear flow.
Colloidal clusters are used as advanced optical materials including microsphere resonators with a WGM and novel building blocks for colloidal crystals with diamond symmetry. It was shown by simulation that tetramer clusters with sticky patches can possibly form diamond symmetry (Zhang et al. 2005).
3.2 Colloidal photonic crystals
Colloidal particles have been studied as atomic and molecular model systems. In a colloidal suspension, the complex interaction among the particles leads to the phase behavior of colloidal system. Analogous to real atomic and molecular systems, a colloidal system also has gas-like, liquid-like and solid (crystal)-like phases according to the interparticle distance and regularity of structures. Different interparticle forces induce various crystalline structures of colloidal crystals, such as a face-centered cubic (fcc) structure, a body-centered cubic (bcc) structure and a hexagonal close-packed (hcp) structure (Kegel et al. 2000; Yethiraj et al. 2003). Recently, the organization of bidisperse or anisotropic particles were studied to achieve unconventional crystalline structures such as diamond structures and CsCl structures, which cannot be accomplished via the self-assembly of homogeneous isotropic particles (Zhang et al. 2004, 2005; Leunissen et al. 2005; Bartlett et al. 2005; Kalsin et al. 2006; Shevchenko et al. 2006; Glotzer et al. 2004).
The periodic array of different refractive index materials has discontinuous energy bands; thus, the electromagnetic wave at a specific frequency cannot penetrate the medium. A colloidal assembly whose lattice parameter is matched with the wavelength of light has a photonic bandgap (Xia et al. 2001). These are known as colloidal photonic crystals. Colloidal self-assembly is the most simple and economic process compared to other fabrication methods. However, the controlled preparation of colloidal photonic crystals is challenging, as the process is usually governed by self-organization. To create actual photonic crystal devices, many approaches have been developed to design and control the shapes and defects. Ordinary opaline photonic crystals with fcc lattices do not have a full photonic band gap but only a few pseudogaps. However, inverse opal structures of dielectric materials with refractive-index higher than 2.85 were studied to grant fully 3D photonic band gaps (Vlasov et al. 2001).
Introducing a fluid medium into the interstices of photonic crystals enables tunable bandgap properties, as the photonic bandgap is a function of the refractive indices of the materials as well as the lattice constant and crystalline structures. Filling the interstices of opaline materials with various fluids of different refractive indices shifts the band gaps by changing the effective refractive index of the structures (Gu et al. 2002). Changes in the lattice constants may be induced by the swelling and deswelling of colloidal photonic crystals by solvents (Fudouzi et al. 2003; Holtz et al. 1997; Foulger et al. 2001). Several researchers have shown that the tunable characteristic of colloidal crystals is applicable for detecting or sensing chemicals and biomolecules. The spectral change caused by swelling or the specific binding of biomolecules can be detected by the naked eye or by a UV–Vis–IR spectrometer. Recently, the increasing demand for lab-on-a-chip devices with built-in photonic/optical functions leads to the complementary combination of photonic crystals with microfluidic chips. Microfluidic parts give tunability and addressability to the built-in photonic crystals, and the photonic crystal grants special photonic functionalities to microfluidic chips. However, the controlled preparation of colloidal photonic crystals inside of microfluidic channels and the characterization of photonic properties are in the early stages of research thus far.
Kim et al. reported the earliest result on colloidal crystallization inside polydimethylsiloxane (PDMS) channels, which have been most widely used as microfluidic channel materials (Kim et al. 1996). Colloidal suspensions were crystallized according to the concentration increase caused by evaporation of the solvent water at the ends of the microchannels. They also created inverted opaline structures with inorganic titania sol by infiltrating a titania precursor into the interstices of opaline structure and subsequently removing the particles (Yang et al. 2001). Although they did not show any fluidic functions, this has been a meaningful attempt for the preparation of patterned colloidal structures inside of microchannels.
Several research groups have fabricated colloidal crystals and their inverted structures inside cylindrical capillaries. Although colloidal crystals in a cylindrical confinement have fcc symmetry, all of the colloidal particles along the surface form the (111) plane of fcc lattices. This implies that the cylindrical colloidal crystals (CylCCs) have a pseudo-gap (L-gap) for the light propagating normal to the surface. Due to the surface curvature, closed packed colloidal array has intrinsic defects and grain boundaries. However, when the diameter of the cylindrical channel is much larger than the colloidal size, the curvature effects on the crystallization becomes negligible.
In 2005, the Kamp et al. (2005) demonstrated a method of optical chromatography using colloidal crystal capillary columns (C4s) (2005). As shown in Fig. 8c, d, the stop bands of the colloidal columns were tuned by filling the colloidal crystals with alkanes such as octane, nonane and decane. Although they have similar molecular structures and molecular weights, a reflectance peak shift was detected from small refractive index variations. It is applicable to a novel liquid-chromatography column with a higher resolution and shorter processing time compared to existing columns.
Likewise, colloidal assemblies of other specific geometries can be self-organized by introducing confining geometries of specific shapes. Planar, cylindrical and spherical geometries are possible for use in different directional properties and applications. The colloidal self-assembly method has an economic advantage in the mass production of photonic crystals compared to top-down approaches. From an optofluidic point of view, colloidal photonic crystals present novel means for optofluidics because the shapes and geometries are easily controllable by manipulating external conditions.
4 Optofluidic integration of colloidal particles and their assemblies
Colloidal particles and their assembled structures endow special optical and photonic functions to microfluidic devices. As colloidal particles have various optical and photonic properties not only for dispersed states but also for aggregates, Combination of a colloidal system with microfluidics is one of the most powerful methods toward integrated optofluidic systems. Integration of colloidal structures enables various optofluidic functions that cannot be realized by fluids alone. However, conventional fluids have many limitations in the properties, such as their dielectric constants, conductivity and non-linear optical effects, and colloidal dispersions. Their assembled structures have various physical properties such as quantum confinement effects, strong light scattering capabilities and photonic band gaps. Chemical and biomolecular functions are combined with a microfluidic system through the surface modification of colloidal particles. Optical signal processing, flow cytometry, and photonic crystal sensors are some of their applications.
Fluorescent colloidal particles have been used in microfluidics, especially in the area of biotechnology. Flow cytometry is an important application which includes in-situ cell counting, fluorescence detection and cell sorting. Instead of a real cell, polymeric colloidal spheres are typically used as representatives to simulate cell motion in a fluid flow. In addition, to reduce total size of cytometric devices, optofluidic integration technology has been developed. Optical fibers have been incorporated with microfluidic channels to collect tiny photon signals from a specified spot. At present, only single function is endowed to a single optofluidic chip. However, various optical and photonic functions that include photo-detectors, optical filters, waveguides, switches, lenses, mirrors and interferometers are ultimate optofluidic integration examples.
Using colloidal particles with a specific refractive index, interferometric signal processing is also possible. According to changes in the optical path length that are determined by the gap distance and refractive index difference with the surrounding medium, phase delay and interference take place. Therefore, in a colloidal system, the size, position and refractive index of the colloidal particles can determine the characteristics of the optofluidic interferometer.
The bandgap shift is visualized by the reflectance spectra of silica and polystyrene colloidal crystals with various solvents in Fig. 12d.
Although only a small number of research groups have reported early stage results of optofluidic integration based on colloidal particles and their assembled structures, this field is continuously expanding due to the unique photonic/optical properties of colloids and their assembled structures.
5 Future vision of colloid-based optofluidics
5.1 LSPR and SERS based devices toward label free detection of molecules
It is well known that discontinuous metal films lead to the formation of strong localized electromagnetic fields. The spectral signal of the localized electromagnetic field shows shifts in response to changes in environmental conditions. The specific binding of molecules or even a small change in the refractive index of the surrounding media induces peak shifts as well as amplification of the spectrum. LSPR and SERS are representative phenomena of electromagnetic wave localization, which enables the label-free detection of specific chemicals and biomolecules.
In general, Raman scattering is not adequate for microanalysis due to the analytical inefficiency associated with it. Therefore, surface-enhanced Raman scattering (SERS) has been suggested as a breakthrough technology to achieve a remarkable advance in the sensitivity level. The Raman intensity with binding molecules can be strongly enhanced within the localized electromagnetic field generated via LSPR excitation of a noble metal surface. To receive the weak Raman signals, conventional Raman instruments require expensive optical systems including high power lasers and extremely sensitive detectors. However, SERS active substrates may offer a huge increment of the signal (106–108) without the need for an expensive system. Although it is certain that researchers using these systems will incur a considerable expense, there remains much room for the development of single molecule detection techniques that require signal amplification of 1014.
5.2 Nanosphere lithography (NSL) for LSPR and SERS active optofluidic chips
Present colloidal sciences have been based on the direct application of colloidal particles and their assemblies. However, to control the photonic properties of colloidal optofluidic systems, it is necessary to synthesize the nanoparticles and their assemblies with various shapes and sizes. Spheres, pyramids, rods and cube-shaped colloidal nanoparticles were prepared by synthetic methods, yet the lithographic approach still has advantages in the formation of ordered arrays with specific shapes, positions and directions. Although electron-beam lithography (EBL) or focused-ion-beam lithography (FIB) enables the creation of designed nanodots and nanopores, practical-scale mass production is highly limited due to the expensive fabrication cost and low throughput rate.
An optofluidic system based on colloidal particles and their assembled structures was developed by utilizing the various optical/photonic properties of colloids. Not only the isolated colloidal particles and their dispersions but also the self-assembled colloidal structures have novel and unique properties. The classical properties of the colloidal system offer rheological effects to the fluid according to the concentration and surface chemistry of the dispersed colloids. Light scattering is an example of a colloid–photon relationship similar to the dynamic light-scattering methods that have long been used to measure the size and distribution of complex fluids from refractive index data. As the colloidal particles are dispersed in a fluid medium, the colloidal system plays a key role in the fluid–photon relationship of optofluidics. Recent dramatic advances in colloidal systems with special photonic properties have enlarged the territory of the optofluidic field. In addition to the dispersions of colloidal particles with photoluminescence, SPR, SERS and photothermal effects, colloidal aggregates and crystals have novel characteristics such as photonic band gaps and abnormal dispersion of light. Moreover, 2D colloidal arrays act as a mask for the lithography of proper substrates by reactive ion etching. Colloidal lithography is one of the most effective methods to the achieving of patterned substrates with LSPR (localized surface plasmon resonance) and SERS activities. Combining colloidal sciences with microfluidics leads to a breakthrough for novel optofluidic devices. Although colloidal dispersion and their assemblies have been studied, their complex assembled structures in fluidic systems have not been intensively studied from an optofluidic point of view. As colloids have a great potential in materials chemistry and biotechnology, the optical and photonic characteristics of optofluidic systems integrated with colloidal system can be applied in various fields of chemistry and biotechnology. There remain ample rooms for advances in optofluidics based on colloidal sciences.
This work was supported a grant from the Creative Research Initiative Program of the Ministry of Science and Technology for “Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems”. The authors also appreciate partial support from the Brain Korea 21 Program.