Niranjan Malvadkar

An engineered anisotropic nanofilm with unidirectional wetting properties

Anisotropic textured surfaces allow water striders to walk on water, butterflies to shed water from their wings and plants to trap insects and pollen. Capturing these natural features in biomimetic surfaces is an active area of research. Here, we report an engineered nanofilm, composed of an array of poly(p-xylylene) nanorods, which demonstrates anisotropic wetting behaviour by means of a pin-release droplet ratchet mechanism. Droplet retention forces in the pin and release directions differ by up to 80 μN, which is over ten times greater than the values reported for other engineered anisotropic surfaces. The nanofilm provides a microscale smooth surface on which to transport microlitre droplets, and is also relatively easy to synthesize by a bottom-up vapour-phase technique. An accompanying comprehensive model successfully describes the films anisotropic wetting behaviour as a function of measurable film morphology parameters.

Bio-organism sensing via surface enhanced Raman spectroscopy on controlled metal/polymer nanostructured substrates

A new class of nonlithographically prepared surface enhanced Raman spectroscopy (SERS) substrates based on metalized, nanostructured poly(p-xylylene) films has been developed and optimized for surface plasmon response with a view to applications of SERS detection of microbial pathogens, specifically, bacteria and viruses. The main emphasis has been on achieving high spot to spot, sample to sample reproducibility of the SERS signals while maintaining useful enhancement factors. The use of these surfaces, metalized with either Ag or Au, provides a noninvasive and nondestructive method for spectral fingerprint analyses of both bacteria and viruses. Examples are given for the detection of bacteria (E. coli and B. cereus) and viruses (respiratory syncytial virus and Coxsackievirus). Our method is able to distinguish Gram positive from Gram negative bacterial strains as well as enveloped and nonenveloped viruses. The results demonstrate the development of a new class of SERS substrates which can provide rapid, selective identification of infectious agents without amplification of cultures.

Quantitative analysis of creatinine in urine by metalized nanostructured parylene.

A highly accurate, real-time multisensor agent monitor for biomarker detection is required for early detection of kidney diseases. Urine creatinine level can provide useful information on the status of the kidney. We prepare nanostructured surface-enhanced Raman spectroscopy (SERS) substrates without template or lithography, which provides controllable, well-organized nanostructures on the surface, for the quantitative analysis of creatinine concentration in urine. We present our work on sensitivity of the SERS substrate to urine samples collected from diabetic patients and healthy persons. We report the preparation of a new type of SERS substrate, which provides fast (<10 s), highly sensitive (creatinine concentration <0.5 microg/mL) and reproducible (<5% variation) detection of urine. Our method to analyze the creatinine level in urine is in good agreement with the enzymatic method.

Control of protein adsorption onto core-shell tubular and vesicular structures of diphenylalanine/parylene.

The self-assembly of peptides, specifically dipeptides, offers numerous advantages for biological applications. We describe an easy, versatile method of fabricating different types of zwitterionic Phe-Phe dipeptide structures (i.e., tubes and vesicles) through solvent-mediated assembly. The stability of the dipeptide structures is increased by thin polymer coatings of poly(chloro-p-xylylene), a PPX film. We also investigated protein adsorption onto PPX-coated peptide tubes and vesicles by varying the thickness of the polymer film.

Noncovalent mechanism for the conformal metallization of nanostructured parylene films.

We describe a rapid, reliable method of preparing nanoporous Ni or Co films using nanostructured poly(chloro-p-xylylene) (nanoPPX) films as templates. The nanoPPX films are vapor deposited onto Si substrates using oblique angle polymerization (OAP), resulting in the formation of an obliquely aligned PPX nanorod array on the substrate. The nanoPPX films are then subjected to noncovalent functionalization using an aromatic ligand (i.e., pyridine) by means of treatment with either an aqueous solution of the ligand or ligand vapor. The results of quartz crystal microbalance and X-ray diffraction studies support a model in which pyridine adsorption is facilitated by the formation of pi-pi interactions with aromatic moieties in the amorphous surface regions of nanoPPX. The physisorbed pyridine in the nanoPPX film can subsequently bind a catalytic Pd(II)-based colloidal seed layer. Continuous, conformal Ni or Co films, characterized by FIB/SEM and AFM, are grown on the Pd(II)-laden nanoPPX films using electroless metallization. Analogous metallization of a conventionally deposited planar PPX film results in noncontinuous or patchy metal deposits. Such behavior is attributed to the sluggish adsorption of pyridine in the planar PPX film, resulting in an approximately 22-fold decrease in the quantity of pyridine adsorbed compared to that in a nanoPPX film. Consequently, the level of Pd(II) bound by pyridine on a planar PPX film is insufficient to catalyze continuous metallization. Results of a statistical two-level factorial design indicate that the morphology of the metal layer formed on a nanoPPX film is profoundly influenced by the ligand adsorption condition (i.e., aqueous ligand vs ligand vapor treatment) and is correlated to the catalytic activity of Co films for the production of hydrogen from sodium borohydride decomposition.

Liquid phase deposition of titania onto nanostructured poly-p-xylylene thin films

We describe a simple, solution-based, two-step process for the fabrication of titania–parylene composite films as a prerequisite for their evaluation as materials for bioimplant applications. In the first step, a ligand capable of binding titania, such as phenylphosphonic acid, is physisorbed onto a nanostructured poly-p-xylylene thin film previously prepared via surface-promoted oblique angle polymerization of radicals formed during vapor-phase pyrolysis of [2.2]-p-cyclophanes. The adsorbed ligand templates conformal growth of titania on the polymer surface in the second step via a liquid phase deposition process involving the controlled hydrolysis of (NH4)2TiF6 in the presence of H3BO3 in pH 2.88 aqueous solution at ∼50 °C. SEM and AFM analyses support a deposition mechanism that includes direct growth of titania on the ligand-impregnated parylene surface, as well as incorporation of titania nanoparticles nucleated in solution into the growing film. XPS and XRD results show that the as-deposited titania contains both amorphous and nanocrystalline anatase phases, with the latter readily consolidated by annealing at ∼200 °C without destruction of the underlying parylene polymer. Titania adhesion can be tuned by proper choice of the ligand, with ligands such as phenylphosphonic acid that strongly bind to titanium dioxide leading to deposition of titania films that pass the Scotch® tape adhesion test.

Functional Nanostructured Polymer- Metal Interfaces

The study of polymer–metal surfaces is important for basic scientific research as well as many practical applications in aircraft, automobile, biomedical, and electronics industries. The possibility of controlling particle size and particle surface chemistry of metals would help us to understand the fundamental mechanism of polymer–metal adhesion in general. We have recently demonstrated that nanostructured polymers can be fabricated by an oblique-angle polymerization method. These structures have a high aspect ratio and the production technique does not require any template or lithography method or a surfactant for deposition. We studied influences of the chemical functionality, morphology, and topology of the nanostructured films on the physical properties of metallic–polymer interfaces. Based on the nanostructured polymer mediated metal technology, we can develop novel polymer–metal interfaces with the following attributes: (1) high surface area materials with controlled roughness, (2) light weight and high adhesion strength of polymer to metal, and (3) industrial-scale deposition.