Sangho Bok

Coatings and surface modifications imparting antimicrobial activity to orthopedic implants

Bacterial colonization and biofilm formation on an orthopedic implant surface is one of the worst possible outcomes of orthopedic intervention in terms of both patient prognosis and healthcare costs. Making the problem even more vexing is the fact that infections are often caused by events beyond the control of the operating surgeon and may manifest weeks to months after the initial surgery. Herein, we review the costs and consequences of implant infection as well as the methods of prevention and management. In particular, we focus on coatings and other forms of implant surface modification in a manner that imparts some antimicrobial benefit to the implant device. Such coatings can be classified generally based on their mode of action: surface adhesion prevention, bactericidal, antimicrobial-eluting, osseointegration promotion, and combinations of the above. Despite several advances in the efficacy of these antimicrobial methods, a remaining major challenge is ensuring retention of the antimicrobial activity over a period of months to years postoperation, an issue that has so far been inadequately addressed. Finally, we provide an overview of additional figures of merit that will determine whether a given antimicrobial surface modification warrants adoption for clinical use.

Femtogram-level detection of Clostridium botulinum neurotoxin type A by sandwich immunoassay using nanoporous substrate and ultra-bright fluorescent suprananoparticles

We report a simple, robust fluorescence biosensor for the ultra-sensitive detection of Clostridium botulinum Neurotoxin Type A (BoNT/A) in complex, real-world media. High intrinsic signal amplification was achieved through the combined use of ultra-bright, photostable dye-doped nanoparticle (DOSNP) tags and high surface area nanoporous organosilicate (NPO) thin films. DOSNP with 22 nm diameter were synthesized with more than 200 times equivalent free dye fluorescence and conjugated to antibodies with average degree of substitution of 90 dyes per antibody, representing an order of magnitude increase compared with conventional dye-labeled antibodies. The NPO films were engineered to form constructive interference at the surface where fluorophores were located. In addition, DOSNP-labeled antibodies with NPO films increased surface roughness causing diffuse scattering resulting in 24% more scattering intensity than dye-labeled antibody with NPO films. These substrates were used for immobilization of capture antibodies against BoNT/A, which was further quantified by DOSNP-labeled signal antibodies. The combination of optical effects enhanced the fluorescence and, therefore, the signal-to-noise ratio significantly. BoNT/A was detected in PBS buffer down to 21.3 fg mL(-1) in 4 h. The assay was then extended to several complex media and the four-hour detection limit was found to be 145.8 fg mL(-1) in orange juice and 164.2 fg mL(-1) in tap water, respectively, demonstrating at least two orders of magnitude improvement comparing to the reported detection limit of other enzyme-linked immunosorbent assays (ELISA). This assay, therefore, demonstrates a novel method for rapid, ultra-low level detection of not only BoNT/A, but other analytes as well.

Influence of silver grain size, roughness, and profile on the extraordinary fluorescence enhancement capabilities of grating coupled surface plasmon resonance

Since the development of fluoroimmunoassays, researchers have sought a method of substantially enhancing fluorescence intensity to extend the limits of detection to new levels of sensitivity. Surface plasmon resonance (SPR) and metal enhanced fluorescence has long been a topic of research and has led to the development of prism- and grating-based SPR systems. However, with the wide coupling range and ease of exciting SPR on plasmonic gratings with a simple microscope objective, they have tremendous potential for revolutionizing the fields of plasmonics, fluorescence, and sensors. In an effort to better understand the influence of grating profile and metal film properties on the extraordinary fluorescence enhancement capabilities of plasmonic gratings, a novel microcontact printing process and different metal deposition techniques were used to fabricate silver gratings with varying grain diameters, roughnesses, heights, and duty cycles using thermal evaporation and RF sputtering. The resulting plasmonic gratings exhibited fluorescence enhancements up to 116× that of dye-coated glass slides using an epifluorescence microscope, much higher than more expensive prism-based SPR systems. This silver grating represents an extraordinary opportunity to quickly and easily enhance fluorescence and widen the detection limits of common fluorescence based assays with little to no equipment modification.

Plasma modification of polymer surfaces and their utility in building biomedical microdevices

Polymers are widely used in micro-systems for biological detection and sensing and to provide easier alternatives for fabrication of Biomedical Micro-devices (BMMDs). The most widely used polymeric system amenable to micro-fabrication is silicone rubber, particularly poly(dimethylsiloxane) (PDMS). The principal advantage that silicone rubber offers is its ability to get replicated with high aspect ratios by micro-molding. In addition to PDMS, other polymer systems, like resists or epoxies, find extensive use in micro-fabrication providing many aspects such as good interlayer bonding, selective patterning, modified physical properties like variable electrical or optical properties, etc. Most polymer systems are amenable to rapid changes in their surface energies as they are exposed to gas plasmas or UV radiation. Such changes can sometimes be reversible and the exposed surfaces can regain their original configuration with time called hydrophobic recovery. In general, polymer surfaces after such external stimuli become constitutionally highly dynamic and this makes them well suited to prominent applications in fabrication of BMMDs. Our group has extensively worked in the area of polymer surface modification by external stimuli and its characterization and in this paper we have attempted to review some of the groups work.

Electrochemical Properties of Carbon Nanoparticles Entrapped in Silica Matrix

Carbon-based electrode materials have been widely used for many years for electrochemical charge storage, energy generation, and catalysis. We have developed an electrode material with high specific capacitance by entrapping graphite nanoparticles into a sol-gel network. Films from the resulting colloidal suspensions were highly porous due to the removal of the entrapped organic solvents from sol-gel matrix giving rise to high Brunauer-Emmett-Teller (BET) specific surface areas (654 m(2)/g) and a high capacitance density ( approximately 37 F/g). An exponential increase of capacitance was observed with decreasing scan rates in cyclic voltammetry studies on these films suggesting the presence of pores ranging from micro (< 2 nm) to mesopores. BET surface analysis and scanning electron microscope images of these films also confirmed the presence of the micropores as well as mesopores. A steep drop in the double layer capacitance with polar electrolytes was observed when the films were rendered hydrophilic upon exposure to a mild oxygen plasma. We propose a model whereby the microporous hydrophobic sol-gel matrix perturbs the hydration of ions which moves ions closer to the graphite nanoparticles and consequently increase the capacitance of the film.

Evaluation of hybrid sol-gel incorporated with nanoparticles as nano paint

A coating with self-cleaning characteristics has been developed using a TiO2/SiO2 hybrid sol-gel, TiO2 nanoparticles and organosilicate nanoparticles (OSNP). A patented technology of the hybrid sol-gel and OSNP was combined with TiO2 nanoparticles to create the surface chemistry for self-cleaning. Two synthesis methods have been developed to prepare TiO2 nanoparticles (NPs), resulting in the enhancement of local paint by the addition of anatase and rutile TiO2 phases. The NPs size as determined by Dynamic Light Scattering (DLS) ranges within of (3-4) and (20-42) nm, which was also confirmed by Scanning Electron Microscopy (SEM). The nanoparticles showed surface charge (zeta-potential, ζ) of +35 and +25.62 mV for the methods, respectively, and ζ values of +41.31 and 34.02 mV for anatase and rutile phases, respectively. The NPs were mixed with the coating solution (i.e., hybrid sol-gel and OSNP) in different concentrations and thin films were prepared by spin coating. Self-cleaning tests were performed usin...

Enhanced DNA Detection Through the Incorporation of Nanocones and Cavities Into a Plasmonic Grating Sensor Platform

In this paper, a novel plasmonic grating sensor platform was developed and tested for feasibility using a lights-ON fluorescence-based DNA assay. The sensor platform combined the fluorescence enhancement of a grating-based plasmonic platform with the electric field intensifying effects of nanoscale cones and cavities. The gratings were made through a microcontact printing process that replicated HD-DVD disks in polymethylsilsesquioxane and coated with a thin gold film. Nanocavities were incorporated into the sensor platform during the printing process and nanocones were incorporated during the 100-nm gold deposition process. Fluorescently tagged single-strand (ss) DNA molecules were immobilized onto the surface and were designed such that the molecules would fluoresce when bound to a complementary sequence. Sensor substrates were imaged after exposure to a mismatched and matched ssDNA to quantify the fluorescence enhancement of the sensor. Exposure to matched ssDNA resulted in fluorescent emission from the grating that was 13.6× brighter than flat gold, while the nanocones and nanocavities were 37.5× and 47.4× brighter than flat gold, respectively.

Enhanced fluorescence through the incorporation of nanocones/gaps into a plasmonic gratings sensor platform

In this article, a novel plasmonic grating sensor platform was developed and tested for feasibility in sensor applications using a “lights-on” fluorescence based DNA sensor. The sensor platform combined the fluorescence enhancement of a grating-based plasmonic platform with the electric field intensifying effects of nano-scale cones and cavities. The gratings were made through a microcontact printing process that replicated HD-DVD discs in polymethylsilsesquioxane (PMSSQ) and coated in a thin gold film. Nanocavities were incorporated into the sensor platform during the printing process and nanocones were incorporated during the 100 nm gold deposition process. Fluorescently-tagged single-strand (SS) DNA molecules were immobilized onto the surface and were designed such that the molecules would fluoresce when bound to a complementary sequence. Sensor substrates were imaged after exposure to a mismatched and matched oligomer to quantify the fluorescence enhancement of the sensor. Much higher fluorescence intensity was observed on all of the plasmonic structures as compared to flat gold.

Novel nanostructured platform and nanoparticles for sensitive detection of biological materials

In this paper, a simple and robust sensing assay platform for detection of biological materials is reported. The fluorescence based sensing platform benefits from the combined use of our novel dye-doped nanoparticle tags and nanoporous high surface area (∼1,400 m2/g) films to achieve high intrinsic signal amplification. The dye-doped nanoparticles are synthesized from poly-methylsilsesquioxane (PMSSQ) encapsulating fluorescent dye molecules that can be conjugated to antibodies and proteins. We have achieved conjugation of up to 64 dye molecules per single antibody, which represents more than an order of magnitude increase in conjugation efficiency compared to conjugation of free dye molecules. Capture of the dye-doped nanoparticle labeled antibodies by nanoporous organosilicate (NPO) films has yielded 540 fold increase in fluorescence response compared to immobilization of dye labeled antibodies on (flat) glass substrates. This assay demonstrates a general way for detection of analytes in very low concentration.

In Situ Characterization of Photothermal Nanoenergetic Combustion on a Plasmonic Microchip

Plasmonic gratings facilitate a robust in situ diagnostic platform for photothermal combustion of nanoenergetic composite thin films using an optical microscope and a high-speed camera. Aluminum nanoparticles (Al NPs) embedded in a fluoropolymer oxidizer are cast onto a plasmonic grating microchip and ignited using a low-power laser. The plasmonic grating enhances both spatial resolution and sufficient photothermal coupling to combust small Al NP clusters, initiating localized flames as small as 600 nm in size. Two-color pyrometry obtained from a high-speed color camera indicates an average flame temperature of 3900 K. Scattering measurements using polarized light microscopy enabled precise identification of individual Al NPs over a large field of view, leading to 3D reconstruction of combustion events.