Ashis Mukhopadhyay

Micro- and nanorheology

‘Microrheology’, the determination of viscoelastic properties of soft media from the observed motion of microscopic-sized tracer particles, has undergone a recent surge of development because it allows access to an unprecedented range of frequency response with unprecedented spatial resolution. Several complementary techniques have been developed, but discrepancies have been found among different experiments especially in complex systems, and the validity of interpretation is under debate. ‘Nanorheology’ after explicit confinement of samples to variable thickness, 1 nm to 1 m, is also enjoying rapid development, particularly nanorheology integrated with spectroscopy. Developments, limitations, and opportunities are discussed. 2001 Elsevier Science Ltd. All rights reserved.

Dynamics of gold nanoparticles in a polymer melt

Gold nanoparticles are used as a luminescent contrast agent to study size-dependent dynamics in polymer matrix. The experiments measured the diffusion coefficient of particles in poly(butyl methacrylate) melt by tracking their motion within a diffraction-limited focus of a laser with 150 fs pulses at 800 nm. Our results indicate that for unentangled polymers, when the particle radius (R) is greater than the gyration radius (Rg) of the chain, the Stokes–Einstein relation can accurately predict particle dynamics. For longer chains, if the entanglement mesh length is larger than R, the particle diffuses ∼250 times faster than predicted by the Stokes–Einstein relation.

Diffusion of Nanoparticles in Semidilute and Entangled Polymer Solutions

We studied the diffusion of gold nanoparticles in semidilute and entangled solutions of polystyrene (PS) in toluene using fluctuation correlation spectroscopy (FCS). The polymer concentration was varied from approximately 6c* to 20c*, where c* is the overlap concentration. In our experiments, the particle radius (R approximately 2.5 nm) was much smaller compared to the radius of gyration (Rg approximately 18 nm) of the chain but comparable to the average mesh size (xi) of the fluctuating polymer network. The diffusion coefficient (D) of the particles decreased monotonically with polymer concentration and it can be fitted with a stretched exponential function, D=D0 exp(-microcnu), with the value of the scaling parameter, nu approximately 0.9. At high concentration of the polymer, a clear subdiffusive motion of the particles was observed. The results were compared with the diffusion of free dyes (coumarin 480), which showed normal diffusive behavior for all concentrations.

Probing Diffusion of Single Nanoparticles at Water–Oil Interfaces

The diffusion of nanoparticles at a water-alkane interface is studied using fluorescence correlation spectroscopy. Hydrophilic and hydrophobic quantum dots of 5, 8, and 11 nm radius are used. A slow-down of nanoparticle diffusion at the liquid-liquid interface is observed. The effect is most evident when the viscosities of both liquid phases are similar, here, at the water-decane interface. In this case, the interfacial diffusion coefficients of the hydrophilic particles are 1.5 times and those of the hydrophobic particles 2 times lower than the corresponding bulk values.

Nanopatterning effects on astrocyte reactivity.

An array of design strategies have been targeted toward minimizing failure of implanted microelectrodes by minimizing the chronic glial scar around the microelectrode under chronic conditions. Current approaches toward inhibiting the initiation of glial scarring range from altering the geometry, roughness, size, shape, and materials of the device. Studies have shown materials which mimic the nanotopography of the natural environment in vivo will consequently result in an improved biocompatible response. Nanofabrication of electrode arrays is being pursued in the field of neuronal electrophysiology to increase sampling capabilities. Literature shows a gap in research of nanotopography influence in the reduction of astrogliosis. The aim of this study was to determine optimal feature sizes for neural electrode fabrication, which was defined as eliciting a nonreactive astrocytic response. Nanopatterned surfaces were fabricated with nanoimprint lithography on poly(methyl methacrylate) surfaces. The rate of protein adsorption, quantity of protein adsorption, cell alignment, morphology, adhesion, proliferation, viability, and gene expression was compared between nanopatterned surfaces of different dimensions and non-nanopatterned control surfaces. Results of this study revealed that 3600 nanopatterned surfaces elicited less of a response when compared with the other patterned and non-nanopatterned surfaces. The surface instigated cell alignment along the nanopattern, less protein adsorption, less cell adhesion, proliferation and viability, inhibition of glial fibrillary acidic protein, and mitogen-activated protein kinase kinase 1 compared with all other substrates tested.

Interaction and diffusion of gold nanoparticles in bovine serum albumin solutions

We studied the interactions of small sized, radius of 2.5–10 nm, gold nanoparticles (AuNPs) with bovine serum albumin (BSA). Fluctuation correlation spectroscopy (FCS) was used to monitor the changes in the Brownian diffusion of these NPs. Our results indicate a BSA monolayer formation at the NP surface with a thickness of 3.8 nm. The thickness of the adsorbed layer was independent of NP size. Best fit was obtained by the anticooperative binding model with the Hill coefficient of n = 0.63. Dissociation constant (KD) increases with particle size indicating stronger interaction of BSA with smaller sized NPs.

An integrated platform for surface forces measurements and fluorescence correlation spectroscopy

We describe an apparatus to measure the diffusion of dilute fluorophores in molecularly thin liquid films within a surface forces apparatus (SFA). The design is a significant modification of the traditional SFA in that it allows one to combine nanorheology with the single-molecule sensitive technique of fluorescence correlation spectroscopy. The primary enabling idea was to place a miniaturized SFA onto the stage of an optical microscope equipped with a long working distance objective and illuminated by a femtosecond laser. A secondary enabling idea was that the silver coating on the backside of mica, normally used in the traditional SFA design for interferometric measurements of the film thickness, was replaced by multilayer dielectric coatings that allowed simultaneous interferometry and fluorescence measurements in different regions of the optical spectrum. To illustrate the utility of this instrument, we contrast the translational diffusion of rhodamine dye molecules (in the solvent, 1,2-propane diol)...

Translational Anisotropy and Rotational Diffusion of Gold Nanorods in Colloidal Sphere Solutions

We studied the translational and rotational diffusion of gold nanorods within a rod/sphere composite liquid using fluctuation correlation spectroscopy (FCS). The nanorods of length L ∼ 60 nm and diameter d ∼ 17 nm were used at a fixed concentration of ∼1 pM. The concentration of colloidal Ludox spheres (size ∼ 26 nm) was varied up to a volume fraction of ϕ = 0.3 or approximately 7 spheres/L(3). Our experiments showed significant translational anisotropy as the sphere concentration was increased. The translational diffusion parallel to the rod axis (D(T∥)) followed very close to the bulk viscosity of the solution. However, the diffusion normal to the rod axis (D(T⊥)) experienced a significantly higher frictional force. For volume fraction ϕ > 0.1 a slightly modified caging theory by Pecora and Deutch could explain the rotational diffusion of the rods very well. At low volume fraction the agreement is poor, which we interpreted as modification of the local ordering of the spheres, which can affect the rod rotation.

Comparison of nanoparticle diffusion using fluorescence correlation spectroscopy and differential dynamic microscopy within concentrated polymer solutions

We studied the diffusion of nanoparticles (NPs) within aqueous entangled solutions of polyethylene oxide (PEO) by using two different optical techniques. Fluorescence correlation spectroscopy, a method widely used to investigate nanoparticle dynamics in polymer solution, was used to measure the long-time diffusion coefficient (D) of 25 nm radius particles within high molecular weight, Mw = 600 kg/mol PEO in water solutions. Differential dynamic microscopy (DDM) was used to determine the wave-vector dependent dynamics of NPs within the same polymer solutions. Our results showed good agreement between the two methods, including demonstration of normal diffusion and almost identical diffusion coefficients obtained by both techniques. The research extends the scope of DDM to study the dynamics and rheological properties of soft matter at a nanoscale. The measured diffusion coefficients followed a scaling theory, which can be explained by the coupling between polymer dynamics and NP motion.

Comparing the activation energy of diffusion in bulk and ultrathin fluid films

We have measured the activation energy (E act) of translational diffusion for a dissolved fluorescent dye in bulk and within an ultrathin liquid film formed on a solid substrate. The experiments were performed using the single-molecule sensitive technique of fluorescence correlation spectroscopy. From the temperature-dependent measurements, we have determined that the activation energy for a few nanometer thick fluid film increases by a factor of approximately 3-4 compared to bulk liquid. The results are confirmed for two distinctly different systems in regard to molecular shape, tetrakis (2-ethylhexoxy) silane and hexadecane.