Hongjun Zeng

Surface functionalization of thin-film diamond for highly stable and selective biological interfaces

Carbon is an extremely versatile family of materials with a wide range of mechanical, optical, and mechanical properties, but many similarities in surface chemistry. As one of the most chemically stable materials known, carbon provides an outstanding platform for the development of highly tunable molecular and biomolecular interfaces. Photochemical grafting of alkenes has emerged as an attractive method for functionalizing surfaces of diamond, but many aspects of the surface chemistry and impact on biological recognition processes remain unexplored. Here we report investigations of the interaction of functionalized diamond surfaces with proteins and biological cells using X-ray photoelectron spectroscopy (XPS), atomic force microscopy, and fluorescence methods. XPS data show that functionalization of diamond with short ethylene glycol oligomers reduces the nonspecific binding of fibrinogen below the detection limit of XPS, estimated as > 97% reduction over H-terminated diamond. Measurements of different forms of diamond with different roughness are used to explore the influence of roughness on nonspecific binding onto H-terminated and ethylene glycol (EG)-terminated surfaces. Finally, we use XPS to characterize the chemical stability of Escherichia coli K12 antibodies on the surfaces of diamond and amine-functionalized glass. Our results show that antibody-modified diamond surfaces exhibit increased stability in XPS and that this is accompanied by retention of biological activity in cell-capture measurements. Our results demonstrate that surface chemistry on diamond and other carbon-based materials provides an excellent platform for biomolecular interfaces with high stability and high selectivity.

Wear-resistant diamond nanoprobe tips with integrated silicon heater for tip-based nanomanufacturing

We report exceptional nanoscale wear and fouling resistance of ultrananocrystalline diamond (UNCD) tips integrated with doped silicon atomic force microscope (AFM) cantilevers. The resistively heated probe can reach temperatures above 600 degrees C. The batch fabrication process produces UNCD tips with radii as small as 15 nm, with average radius 50 nm across the entire wafer. Wear tests were performed on substrates of quartz, silicon carbide, silicon, or UNCD. Tips were scanned for more than 1 m at a scan speed of 25 mum s(-1) at temperatures ranging from 25 to 400 degrees C under loads up to 200 nN. Under these conditions, silicon tips are partially or completely destroyed, while the UNCD tips exhibit little or no wear, no signs of delamination, and exceptional fouling resistance. We demonstrate nanomanufacturing of more than 5000 polymer nanostructures with no deterioration in the tip.

Rapid thermal lysis of cells using silicon–diamond microcantilever heaters

This paper presents the design and application of microcantilever heaters for biochemical applications. Thermal lysis of biological cells was demonstrated as a specific example. The microcantilever heaters, fabricated from selectively doped single crystal silicon, provide local resistive heating with highly uniform temperature distribution across the cantilevers. Very importantly, the microcantilever heaters were coated with a layer of 100 nm thick electrically insulating ultrananocrystalline diamond (UNCD) layer used for cell immobilization on the cantilever surface. Fibroblast cells or bacterial cells were immobilized on the UNCD/cantilever surfaces and thermal lysis was demonstrated via optical fluorescence microscopy. Upon electrical heating of the cantilever structures to 93 degrees C for 30 seconds, fibroblast cell and nuclear membrane were compromised and the cells were lysed. Over 90% of viable bacteria were also lysed after 15 seconds of heating at 93 degrees C. This work demonstrates the utility of silicon-UNCD heated microcantilevers for rapid cell lysis and forms the basis for other rapid and localized temperature-regulated microbiological experiments in cantilever-based lab on chip applications.

A quantitative study of detection mechanism of a label-free impedance biosensor using ultrananocrystalline diamond microelectrode array.

It is well recognized that label-free biosensors are the only class of sensors that can rapidly detect antigens in real-time and provide remote environmental monitoring and point-of-care diagnosis that is low-cost, specific, and sensitive. Electrical impedance spectroscopy (EIS) based label-free biosensors have been used to detect a wide variety of antigens including bacteria, viruses, DNA, and proteins due to the simplicity of their detection technique. However, their commercial development has been hindered due to difficulty in interpreting the change in impedance upon antigen binding and poor signal reproducibility as a result of surface fouling and non-specific binding. In this study, we develop a circuit model to adequately describe the physical changes at bio functionalized surface and provide an understanding of the detection mechanism based on electron exchange between electrolyte and surface through pores surrounding antibody-antigen. The model was successfully applied to extract quantitative information about the bio surface at different stages of surface functionalization. Further, we demonstrate boron-doped ultrananocrystalline diamond (UNCD) microelectrode array (3 × 3 format, 200 μm diameter) improves signal reproducibility significantly and increases sensitivity by four orders of magnitude. This study marks the first demonstration of UNCD array based biosensor that can reliably detect a model Escherichia coli K12 bacterium using EIS, positioning this technology for rapid adoption in point-of-use applications.

Nanofabrication of sharp diamond tips by e-beam lithography and inductively coupled plasma reactive ion etching

Ultrasharp diamond tips make excellent atomic force microscopy probes, field emitters, and abrasive articles due to diamond’s outstanding physical properties, i.e., hardness, low friction coefficient, low work function, and toughness. Sharp diamond tips are currently fabricated as individual tips or arrays by three principal methods: (1) focused ion beam milling and gluing onto a cantilever of individual diamond tips, (2) coating silicon tips with diamond films, or (3) molding diamond into grooves etched in a sacrificial substrate, bonding the sacrificial substrate to another substrate or electrodepositing of a handling chip, followed by dissolution of the sacrificial substrate. The first method is tedious and serial in nature but does produce very sharp tips, the second method results in tips whose radius is limited by the thickness of the diamond coating, while the third method involves a costly bonding and release process and difficulties in thoroughly filling the high aspect ratio apex of molding groo...

Charging characteristics of ultra-nano-crystalline diamond in RF MEMS capacitive switches

Modifications to a standard capacitive MEMS switch process have been made to allow the incorporation of ultra-nano-crystalline diamond as the switch dielectric. The impact on electromechanical performance is minimal. However, these devices exhibit uniquely different charging characteristics, with charging and discharging time constants 5–6 orders of magnitude quicker than conventional materials. This operation opens the possibility of devices which have no adverse effects of dielectric charging and can be operated near-continuously in the actuated state without significant degradation in reliability.

Characterization of ultrananocrystalline diamond microsensors for in vivo dopamine detection

We show the technical feasibility of coating and micro patterning boron-doped ultrananocrystalline diamond (UNCD®) on metal microwires and of applying them as microsensors for the detection of dopamine in vivo using fast-scan cyclic voltammetry. UNCD electrode surface consistently generated electrochemical signals with high signal-to-noise ratio of >800 using potassium ferrocyanide-ferricyanide redox couple. Parylene patterned UNCD microelectrodes were effectively applied to detect dopamine reliably in vitro using flow injection analysis with a detection limit of 27 nM and in the striatum of the anesthetized rat during electrical stimulation of dopamine neurons.

High-Frequency Thin-Film AlN-on-Diamond Lateral–Extensional Resonators

In this paper, low-impedance lateral-extensional microresonators are fabricated on a stack of aluminum nitride (AlN) directly deposited on a polished ultrananocrystalline diamond (UNCD) film. The large acoustic velocity of UNCD is utilized to extend the frequency of such resonators beyond 1 GHz while the frequency-defining features are not reduced excessively. In order to promote the growth of a c-plane piezoelectric AlN film, the surface of the UNCD film is polished after deposition. Three different UNCD films with different Youngs modulus values were prepared, and frequencies up to two times that of similar devices fabricated on silicon have been achieved. The finite-element analysis is employed to evaluate the effect of various physical parameters on the performance of the thin-film piezoelectric-on-substrate resonators in order to achieve very low motional resistance (Rm). Several resonators were designed with various lateral dimensions and different numbers of support tethers to evaluate the propositions. The lowest Rm was measured from a multitethered 29th-order thin-film piezoelectric-on-diamond (TPoD) resonator (22 Ω) and f · Q product of 2.72 * 1012 at 888 MHz. The temperature coefficient of frequency of this TPoD resonator is measured to be -9.6 ppm/°C, which is much lower than that of the devices fabricated on silicon. Also, this device can withstand input powers up to +27 dBm, leading to a delivered power density per unit area of ~2.9 μW/μm2.

Very low-loss high frequency lateral-mode resonators on polished ultrananocrystalline diamond

For the first time, high frequency (∼900MHz) lateral-mode thin-film piezoelectric-on-diamond resonators are reported with insertion loss values as low as 2.6dB when terminated by 50Ω impedance. In this work, the surface of the as-deposited ultrananocrystalline diamond (UNCD) is polished to promote the direct growth of highly oriented c-plane AlN which is critical in enhancing the electromechanical coupling and therefore reducing the motional impedance of the resonator. The frequency of resonators fabricated on 3–4um UNCD with a 930GPa Youngs modulus is measured to be about 2× higher than similar devices fabricated on 5um of silicon. These resonators exhibit a low temperature coefficient of frequency (TCF) of −9.6 ppm/°C and have a great potential for being utilized in channel-select high frequency filters [1].

Piezoresistive Microcantilevers From Ultrananocrystalline Diamond

This paper reports on the temperature-dependent electrical resistivity and piezoresistive characteristics of boron-doped ultrananocyrstalline diamond (UNCD) and the fabrication of piezoresistive microcantilevers using boron-doped and undoped UNCD. The devices consist of 1-μm-thick doped UNCD on either 1- or 2-μm-thick undoped UNCD. The electrical resistivity of doped UNCD is 0.1 Ω · cm at room temperature, which is five orders of magnitude smaller than the electrical resistivity of undoped UNCD. Over the temperature range of 25°C-200vC, the doped UNCD has a temperature coefficient of electrical resistance of (-1.4 × 10^-3) per °C. The doped UNCD exhibits a significant piezoresistive effect with a gauge factor of 7.53 ± 0.32 and a piezoresistive coefficient of 8.12 × 10^-12 Pa^-1 at room temperature. The piezoresistive properties of UNCD are constant over the temperature range of 25°C-200°C. Microcantilevers having a length of 300 μm have a deflection sensitivity of 0.186 mΩ/Ω per micrometer of cantilever end deflection. These measurements of electrical and piezoresistive properties of doped UNCD could aid the design of future diamond microsystems.