Nanotechnology Lecture 4/12/04 Ways to create or manipulate nanostructures nanolithography dielectrophoresis (applications) optical tweezers (applications) 2D-phase separation Nanodevices nanowires 2D-photonic crystals nanoscale optical sensors Ways to study nanostructures atomic force microscopy surface plasmon resonance
Nano-Patterned Surfaces Two-dimensional phase separation can be used to self-assemble a variety of microstructures at a surface. Thin films of polymer solutions or melts can become unstable and either de-wet the surface or undergo phase separation. Three main types of polymer films have received the most attention: homopolymers A-A-A-A-A-A-A-A-A blends of homopolymers A-A-A-A-A-A-A-A-A and B-B-B-B-B-B-B-B-B block copolymers (A-A-A-A-A-A)-(B-B-B-B-B-B-B)
Homopolymers The basic physics is best illustrated for homopolymers. Polymer solutions or melts are spread on a surface by dipping or spin- coating (allows precise thickness control). When the spreading coefficient S is negative and the Hamaker constant AS/F/V is positive, the film is unstable and dewets via a “spinodal decomposition” mechanism. The interplay between dewetting (causing “roughening” of the film surface) and capillary forces (favoring a smooth surface) results in the film becoming unstable on a characteristic dimension, proportional to the square of the original film thickness and the square-root of the surface tension.
Spinodal decomposition leads to phase separation on a characteristic length scale . spontaneous surface fluctuations…. shorter wavelengths are opposed by surface tension h h x x longer wavelengths are un- favorable at high instability h h x x
Spinodal-like Pattern Formation Fluctuations in film thickness terminate in the formation of holes in the film, eventually leaving isolated droplets of polymer (AFM image).
Blends of In-compatible Homopolymer Even slightly unfavorable enthalpic interactions between constituents of a polymer blend will lead to phase separation in 3D or 2D. In the absence of preferential wetting of the surface by one of the polymer components, 2D, lateral phase separation occurs with the minority component occurring as patches surrounded by majority component. The patch size depends on the viscosity and interfacial free energy between phases, but is typically on the micron scale. The polymer component more highly swollen with solvent will shrink more as the film dries, leaving a depression relative to the less swollen component. As a result, patches appear as islands or holes ~ 100 nm high/deep. PMMA PVP PS substrate
In-compatible Homopolymer Blends (no preferential wetting) Upon increasing the fraction of minor component, the holes/islands percolate to give a bicontinuous pattern. Reversal of roles by the minor/major components leads to holes reverting to islands or vice- versa. The progression of patches of one component increasing in number, going through a bicontinuous morphology, and then inverting to patches of the other component is shown in these AFM images. Length scale is still controlled by fluid viscosity/interfacial energy. .
In-compatible Homopolymer Blends (withpreferential wetting) When one component preferentially wets the substrate, the symmetry is broken and a lamellar phase structure develops - the wetting component forming an enriched layer at the surface and an adjacent depleted layer enriched in the other component of the blend - via a process called “surface-directed” spinodal decomposition. wetting component non-wetting wetting component substrate unstable substrate
substrate In-compatible Homopolymer Blends (withpreferential wetting) Ideally, the fractions of the components and the overall film thickness would allow for the preferentially wetting components to be at the substrate and air interface. The central lamellar layer can then be too thin and develop fluctuations which grow via Rayleigh instability. These capillary fluctuations pinch off the central layer into droplets/ islands with characteristic dimensions. substrate lateral phase separation
Internally Incompatible Blocks Unfavorable interactions between A and B blocks give local phase separation into distinct A -rich and B-rich regions. Because blocks are attached to one-another, the size and shape of these regions depends on the absolute and relative sizes of the blocks. Symmetric blocks (A/B of the same size) phase separate into a lamellar stacking of alternating A and B layers oriented parallel to the substrate. The layer thickness will scale with radius of gyration of the blocks (n). For asymmetric blocks (A > B), the minority block (B) is confined to first cylindrical and then spherical aggregates. . L
Incommensurability in Two-Dimensions Refers to a mismatch between the applied film thickness and the natural layer thickness - determined by the block size. If the film is not some multiple of the lamellae repeat distance, parallel orientation is frustrated, so that lamellar orientation develops running normal to the substrate. Asymmetric di-blocks would normally form hexagonally close-packed cylinders oriented parallel to the surface. But if the cylinder diameter exceeds the layer thickness, then orientation is forced normal to the substrate. .
Incompatible Blocks with Preferential Wetting The lower surface tension block prefers the air/film interface. If the other block can reside at the substrate/film interface, then the lamellae can orient parallel to the surface. Commensurability between the layer thickness and the inter-lamellar spacing controls the surface topology. When the film can accommodate an exact number of lamellar repeat units, the film will be smooth and surface chemically homogeneous. When the film thickness lies between one of these periodic values , the extra increment in thickness undergoes lateral phase separation into islands consisting of full lamellar layers and “holes”. The surface fraction of islands increases as the increment in thickness approaches that of an additional lamellar layer. Thus islands, then bicontinuous, and then dispersed holes occur.
Incompatible di-block copolymer with preferential wetting. Average excess (>nL) film thickness between L/2 and L, forms extra layer of thickness L with “holes” accounting for missing volume Average excess film thickness between L/3 and L/2, “holes” percolate to give bicontinuous morphology Average excess film thickness less than L/3, breaks-up into “islands” of thickness L
Summary of 2-D Phase Separation in Films Can be used to create periodic surface features on micron length sales with height variations on the order of 10-100 nm. Can be used to create periodically inhomogeneous surface chemistry (in terms of wettability or reactivity). Generally requires a specially prepared substrate. The majority of work has been done with organic soluble polymers. Water-based systems need to be developed. Applications to gas separating membranes, photodiodes and photo- voltic devices, light-emitting diodes, antireflective coatings, and to increase the bio-compatibility of implanted medical devices.
(-) (+) Dielectrophoresis - a way to move nano-particles Defined as the lateral motion imparted on uncharged particles as a result of polarization induced by a non-uniform electric field. How does it arise? Particles suspended in a medium will generally have a different dielectric permeability, = ’ + i”, than that medium. If so, then an applied electric field will induce charges to appear at the particle / medium boundary and the creation of an electric dipole moment. (+) (-) When P > m, the particle is polarized more easily than the medium by the external field and the induced dipole moment is aligned with the external field. The dipole moment [ = (q) d ] can be quite large because the induced charge q is separated by the particle dimension d (much larger than a typical molecular dimension).
(+) (-) (+) (-) Dielectrophoresis - a way to move nano-particles When P < m, the particle is not as easily polarized as the medium and the induced dipole opposes the external field. As the frequency of the applied field is increased, it eventually becomes difficult to reverse the polarization within the particle. However, the effects persists even beyond 50 MHz. Necessity of a non-uniform electric field In a parallel plate capacitor (uniform electric field) the total force on a dipole is zero F = (-) E(0) + (+) E(d) = E (- + +) = 0 (-) (+) d
Dielectrophoresis - necessity of non-uniform field But in an electric field gradient, such as is found with a pin and plate electrode set, have E(0) > E(d), so that F = (-) E(0) + (+) E(d) and the particle will migrate along a field line towards (in this case) the higher field of the pin electrode. (+) (-) (+) (-) Basis for separation of colloidal particles Bacteria - gram-positive bacteria have cell walls composed of open networks which are more polarizable than gram-negative bacteria (lipid/protein cell walls). So gram-positives are attracted towards high fields, gram-negatives repulsed. Viable / non-Viable cells - non-viable cells have degraded membranes with unregulated ion diffusivity (high polarizable), while viable cell membranes actively resist non-specific ion diffusion.
Application - Microwires = wires of micron or sub-micron cross-section Dielectrophoresis can be used to self-assemble microwires from colloidal nanoparticles suspended in water. Hermanson et al. (Science294 1082 (2001)) have applied an alternating voltage to a suspension of gold nano- particles, 15-30 nm in dimension. Application of the field allows formation of thin metallic fibers which span the gap, growing out from each electrode at up to 50 m/s. E Planar electrodes 50-250 V AC 50-200 Hz E ~ 250 V/cm 2 - 5 mm gap Advantages of the process Alternating voltage allows particle manipulation without the complications of electro-osmotic or electrochemical (dc) effects. Structures can be assembled in situ, to create “wet” electronic circuits
Application - Microwires Controlling factors: Field strength - must exceed a threshold to overcome electrostatic repulsion between particles. Nanoparticle concentration - must exceed a threshold to encourage agglomeration of particles. Electrolyte level - higher ionic strengths enhance particle agglomeration current (mA) Self-repairing - at high applied voltage, the wires burn out, only to spontaneously re-build themselves by agglomeration of new nano- particles time (s) Performance: Simple ohmic behavior: I V with resistivity of 10-5 to 10-6 ohm m. Coated wires possible - grow in mixed suspension of metallic particles and polystyrene latex spheres - get metallic core surrounded by shell of polymer Act as chemical sensors - adsorption of trace agents from solution affects resistivity because of high wire surface area.
Application - 2D Photonic Crystals Photonic crystals have a periodicity comparable to the wavelength of light - used to manipulate light on the microscale. They can be self-assembled as a 2D colloidal crystal between the gap of a gold planar electrode cell. Lumsdon et al. (Appl. Phys. Lett.82 949 (2003)) report that the single crystal forms rapidly (< 30 s) and reversibly. (+) (-) diffraction hexagonal diffraction pattern Charged polystyrene microspheres (0.5 m) form a densely packed monolayer. The scattering angle is controlled by the interparticle interaction: Separation in water > 10-4M NaCl > 10-3M NaCl
Nanoscale Optical Biosenors Biosensors for the diagnosis and monitoring of diseases, drug discovery, proteomics, and environmental detection of biological agents are in great demand. Fundamentally, a biosensor is derived from the coupling of a ligand-receptor binding reaction to a signal transducer. Optical sensors based on evanescent electromagnetic fields, particularly those based on propagating surface plasmon polaritons (SPP) at planar gold surfaces, are fast becoming the method of choice. ligand receptor Ligand /receptor binding triggers a detectable change in the receptor
Nanoscale Optical Biosenors Surface plasmon resonance (SPR) can be used to monitor a wide range of analyte-surface binding interactions, such as the adsorption of small molecules and ligand-receptor binding. The sensing mechanism of SPR is based on the measurement of small changes in the refractive index that occur in response to the analyte binding at or near the surface of noble metal (Au, Ag, Cu) thin films. Chemo/Bio Sensors based on SPR spectroscopy have many advantages; - refractive index sensitivity of 1 part in 106, corresponding to an areal mass sensitivity of 10-1 pg/mm2. - A sensing length scale determined by the exponential decay of the evanescent electromagnetic field ~ 200 nm. - Multiple modes of detection: angle shift, wavelength shift, imaging - Real-time detection on the 0.1-103 s time scale for measurement of binding kinetics. - Lateral spatial resolution of up to 10 microns.
Surface Plasma Resonance Spectroscopy What it is - An electron charge density wave phenomenon arising when light is reflected from the surface of a metallic film under specific conditions The resonance is a result of energy and momentum being transferred from incident photons to surface plasmons - collective oscillations of conduction free electrons in metals. It is very sensitive to the refractive index of the medium on the opposite side of the film from which reflection occurs.
The Surface Plasma Resonance Effect At the interface between two non-absorbing media of refractive indices n1 and n2, with n2 < n1: n2 (air) < n1 n1 (glass) Light incident at the interface from the medium of higher index will undergo total internal reflection if the incident angle is above a critical angle. The light reflects back into the higher index medium but leaks an electrical field intensity called an evanescent field wave (efw) into the low index medium.
The Role of the Metallic Film The amplitude of the evanescent field wave decreases exponentially with distance into the lower index medium, with a penetration length on the order of the wavelength of the incident light: plasmon, ksp n2 (air) gold film efw, kx n1 (glass) If the n2/n1 interface is coated with a conducting material - like gold or silver, a component of the evanescent field wave may penetrate the metal layer and excite electromagnetic surface plasmons propagating within the conductor surface at the air (n2) interface.
The Resonance Condition The plasmon will enhance the evanescent wave, enhancing its penetration depth. The magnitude of the wavevector kx of the efw depends on the local refractive index and the angle of incidence as: kx = (2/) n1 sin The wavevector of the surface plasmon wave, for a gold film (ngold) and sample medium n2 , is ksp = (2/) [(ngold2 n22)/ (ngold2 + n22)] Thus as the incident angle or the wavelength is varied, kx can cross ksp, at which point the efw field wave will excite the surface plasmon . As a result, there will be a dip in the reflected intensity. reflected intensity
SPR as a Sensitive Detector The gold surface can be used as a detector for adsorbable species in the medium n2, which can be air or a solution. Adsorption of proteins, for instance, from solution onto the gold surface changes the local n2 (within the range of the efw) and hence shifts the reflection angle (or alternatively the wavelength) for the resonance condition. reflected intensity shift in resonance angle upon adsorption The greater the extent of adsorption, the greater the change in n2 and the larger shift in angle. Can sense femto-molar adsorption as well as conformational changes in the adsorbed molecular layer.
Fabrication of Triangular Silver Nanoparticles Triangular silver nanoparticles can be fabricated by nano-lithography. A stable gold colloid is first prepared with a particle size of about 15 nm. A few micro-liters of this colloid is drop coated onto a glass coverslip and dried to give a single-layer colloidal crystal mask of Ag nanoparticles (hexagonally close packed). A thin gold film is then vapor deposited, contacting the glass only in the gaps between the hexagonal packing. The nanosphere mask is then removed by sonication to leave triangular shaped silver deposits of 50 nm height and 100 nm width. Ag vapor sonicate Dimensions controlled by colloid particle size and vapor deposition time.
Functionalization of Silver Triangles The silver surface is covered with self-assembled monolayers of 11- mercaptoundecanoic acid (-COOH tipped) and/or 1-octanethiol (-CH3 tipped). HOOC CH3 CH3 biotin SH SH SH SH glass prism silver The carboxyl moieties are reacted at low levels with biotin to give a biotinylated silver surface - about 100 sites per nanoparticle
, ’, ’ Example of Nanoscale Affinity Biosensing Streptavidin shows an extremely high binding affinity to biotin (Ka ~1013 M-1). Contacting very dilute streptavidin solution with the functionalized surface results in specific binding, causing a subtle change in the local refractive index of the medium surrounding the nano-triangle CH3 CH3 streptviden SH SH SH SH glass prism silver The resonance condition for the nano-triangle with bound streptavidin changes. Detect via localized plasmon resonance as a shift in wave- length of peak absorption.
Sensitivity of Nanosensor Array The LSPR nanosensor operates by detecting refractive index changes within the localized electromagnetic fields surrounding the nanoparticles. Although 100 nM strepavidin gives a saturation wavelength shift of 27 nm (max), 1 pM already gives a reproducible 4 nm red shift. The reason for this non-linear behavior lies in the nature of the binding constant. The relative wavelength shift ( /max)measured versus a range in streptavidin concentration (10-15 to 10-6 M) very nicely follows a Langmuir isotherm with Ka = 1011 M-1. 1.0 /max 0.0 -15 -10 -5 log [strepavidin]
The Future Promises Greater Sensitivity Currently can detect down to 1 pM for high affinity binding. For high through- put screening applications, ultimately aim for single nano particle interrogation so that adjacent nano-triangles can be sensitized for different species. Complex mixtures could then be completely and simultaneously analyzed for each component with the limit of detection tending to single molecules. high affinity low affinity
Typical Experimental Setup detector laser source prism metal film adsorbed film aqueous solution
Optical Tweezers - a manipulation tool in nano-technology Use light to manipulate microscopic objects in the size range from a few nanometers up to about a micron. A strongly focused laser beam is used to catch and hold dielectric particles. The use of optical traps was first introduced by scientists at Bell Laboratories in 1984. specimen plane How they work: laser objective optical trap A laser is focused by a microscope objective to a spot in the specimen plane. Usually, an infrared laser is chosen to minimize sample damage.
Light possess momentum and can exert a pressure All light consists of photons that each have a momentum p, whose magnitude is p = h/ and whose direction is that of propagation. The intensity of light is the number of photons passing a given area A in unit time as given by the Poynting vector S. The momentum flux from light of this intensity is d [d P/dt] = (n/c) S dA The radiation pressure is the force exerted per unit area on an object due to its interaction with light. When light is reflected from or refracted by an object, the momentum of the light changes. The total force on the object is the difference between the momentum flux entering the object and that leaving the object F = (n/c) (Sin - Sout) dA
Light exerts a force on objects it encounters Reflection: With normal incidence on a mirror, Sin = - Sout, F = 2(n/c) Sin dA Thus objects are pushed by the reflection of light from their surface. For 100% reflection from a 60 W light bulb, F = 2(n/c) W = 4 x 10-7 N ! Only objects weighing less than 1 g can feel this force. Sin Sout
Light exerts a force on objects it encounters Refraction: Any change in the direction of light by refraction from an object will change the momentum of light. The object must undergo an equal and opposite momentum change. pparticle Plight pparticle Plight polystyrene bead nbead (1.55) > nmedium (1.33) Pnet So, objects are pulled towards the path of the incident light by refraction!
In an optical tweezer - particles are pulled towards the focus The gaussian incident intensity of the source is brighter in the center than at the edge. Thus the refraction of light from the portion of the particle nearest the center pulls strongly towards the center of the beam while refraction of the light from the edges of the source pulls only weakly away from the center. Problem: reflective particles can be pushed out of the trap. This tendency can be overcome by using dual beam traps in which a particle is illuminated from the front and back so that the reflective pushes cancel out.
What’s it like inside an optical trap? Dielectric (polystyrene) particles in a trap feel a restoring force towards the center of the focused laser. This restoring force is proportional to the distance between the focal point and the particle center - it behaves like an optical spring. x Restoring force F = x where is the stiffness of the trap So, measuring the displacement of the particle (eg., by projecting the image of the bead onto a quadrant photo-diode) determines the displacing force acting on the particle (if is known). Since x can be measured with an accuracy of 10 nm and is typically 50 pN/m, a force resolution of 0.5 pN is possible.
Measuring small forces with optical tweezers Before quantitative measurements can be made, the trap must be calibrated. This is usually done in one of two ways: Viscous drag :The force due to viscous drag on a dielectric sphere of radius r is Fvis = 6rv , where v is the relative velocity of the sphere in a medium of viscosity . Thus if the fluid surrounding a particle fixed in a trap is driven at a fixed flow rate, Fvis is known and the trap force constant can be extracted from the particle displacement x. no flow flow velocity v Fvis = x displacement x 0 0
Measuring small forces with optical tweezers Brownian motion : The Brownian motion of a trapped sphere is described by the Langevin differential equation 6r dx/dt + x = F(t) where F(t) is the stochastic force due to thermal motion. Although this random force has an average value of zero, the modulus of the net displacement x is non-zero and can be used to calibrate . Assuming that the trapping potential is harmonic ( V(x) = 1/2 x2), x2 = kBT/ displacement x2 0 0
Applications to Biochemistry Attach a single DNA molecule to a polystyrene bead coated with streptavidin (DNA labelled at one end with biotin) and suspend the bead in an optical trap. Molecular motors : RNA polymerase is an enzyme which copies DNA sequences to make a single-stranded messenger RNA (mRNA) in the process known as transcription. The mRNA is then used by the ribosome to create a specific protein. Energy is consumed in the process, by which the polymerase moves along the DNA strand. Thus RNA polymerases are molecular motors - using energy to create motion and generating forces in the cell. Optical tweezers can be used to sense these forces.
Molecular Motors RNA polymerase-bound bead is moved to stretch DNA, exerting a force opposing transcription. At high enough displacement (tension in DNA strand), the molecular motor stalls! RNA polymerase is a strong motor, exerting forces up to 25 pN. mRNA DNA RNA polymerase optically trapped bead glass micro-pipette move to stretch DNA
Unzipping DNA / RNA RNA transcription and DNA replication require the double-stranded DNA (ds-DNA) to be converted into ss-DNA (splitting the double helix) - called the “helix-to-coil transition”. Stretch a ds-DNA molecule between two polystyrene beads: B B biotin streptavidin-coated bead
Unzipping DNA / RNA Stretching beyond a contour length of 0.34 nm / base pair encounters a steeply rising force curve. But allowing one end of the DNA to rotate freely gives a cooperative “over-stretching” transition at about 65 pN. Very little additional force is necessary to stretch the molecule to 1.7 times its contour length - corresponding to a force- induced melting in which the base pairs holding the two DNA strands together break as the DNA unwinds. 100 ds-DNA overstretching transition force (pN) ss-DNA DNA extension per base pair (nm) 0 0.2 0.4 0.6