Single-beam plasma source deposition of carbon thin films.

A single-beam plasma source was developed and used to deposit hydrogenated amorphous carbon (a-C:H) thin films at room temperature. The plasma source was excited by a combined radio frequency and direct current power, which resulted in tunable ion energy over a wide range. The plasma source could effectively dissociate the source hydrocarbon gas and simultaneously emit an ion beam to interact with the deposited film. Using this plasma source and a mixture of argon and C2H2 gas, a-C:H films were deposited at a rate of ∼26 nm/min. The resulting a-C:H film of 1.2 µm thick was still highly transparent with a transmittance of over 90% in the infrared range and an optical bandgap of 2.04 eV. Young's modulus of the a-C:H film was ∼80 GPa. The combination of the low-temperature high-rate deposition of transparent a-C:H films with moderately high Young's modulus makes the single-beam plasma source attractive for many coatings applications, especially in which heat-sensitive and soft materials are involved. The single-beam plasma source can be configured into a linear structure, which could be used for large-area coatings.

Indium Tin Oxide Film Characteristics for Cathodic Stripping Voltammetry.

The combination of conductivity, optical transparency, and wide anodic potential window has driven significant interest in indium tin oxide (ITO) as an electrode material for electrochemical measurements. More recently, ITO has been applied to the detection of trace metals using cathodic stripping voltammetry (CSV), specifically manganese (Mn). However, the optimization of ITO fabrication for a voltammetric method such as CSV is yet to be reported, nor have the microstructural properties of ITO been investigated for CSV. Furthermore, CSV does not require optical transparency, thereby allowing nontransparent substrates to be used for deposition. This enables microfabrication procedures to be expanded and simplified compared to glass or quartz. Combining this with the profound importance of sensitive, selective detection of toxic metal ions in environmentally and biologically relevant samples makes ITO especially attractive. In this work, we report a thorough investigation of ITO deposition and processing on silicon (Si) substrates for CSV analysis using Mn as the model analyte. Several ITO process parameters were examined such as heated deposition and post-process annealing. Each ITO film was characterized using a variety of surface, bulk (X-ray diffraction), and electrochemical measurements. Although each ITO film type showed electrochemical activity, the heated and annealed (HA) ITO fabrication process yielded superior results for Mn CSV; a limit of detection (LOD) of 0.1 ppb (1.8 nM) was obtained. This work exemplifies new applications of ITO as an electrode material while providing a baseline for trace detection of toxic metals and other contaminants amenable to detection by CSV.

Isatin Detection Using a Boron-Doped Diamond 3-in-1 Sensing Platform.

Boron-doped diamond (BDD) is a promising electrochemical tool that exhibits excellent chemical sensitivity and stability. These intrinsic advantages coupled with the material's vast microfabrication flexibility make BDD an attractive sensing device. In this study, two different 3-in-1 BDD electrode sensors were fabricated, characterized, and investigated for their capability to detect isatin, an anxiogenic indole that possesses anticonvulsant activity. Each device was comprised of a working, reference, and auxiliary electrode, all made of BDD. Two different working electrode geometries were studied, a 2 mm diameter macroelectrode (MAC) and a microelectrode array (MEA). The BDD quasi-reference electrode was studied by measuring its potential against a traditional Ag/AgCl reference electrode. While the potential shifted as a function of solution pH, a miniscule potential drift was observed when holding the solution pH constant. Specifically, the BDD quasi-reference electrode had a potential of -0.2 V (vs Ag/AgCl) in a pH 7 solution, and this remained stable for a 30-h time period. For the detection of isatin, solutions were analyzed using both sensors in pH 7.4 phosphate buffered saline (PBS). Using the MEA sensor, the limit of detection (LOD, (3σ)/m) for isatin was found to be 0.04 µM; an increase to 0.22 µM was observed with the MAC sensor. These results were compared to those obtained from UV-vis spectrophotometry, where a 0.57 µM LOD was observed. The feasibility for use in a complex sample matrix was also examined by completing measurements in urine simulant. The results presented herein indicate that both 3-in-1 BDD sensors are applicable at low limits of detection with potential application as an electrochemical detector for chromatographic methods.

Highly sensitive detection of urinary cadmium to assess personal exposure.

A series of Boron-Doped Diamond (BDD) ultramicroelectrode arrays were fabricated and investigated for their performance as electrochemical sensors to detect trace level metals such as cadmium. The steady-state diffusion behavior of these sensors was validated using cyclic voltammetry followed by electrochemical detection of cadmium in water and in human urine to demonstrate high sensitivity (>200 µA ppb(-1) cm(-2)) and low background current (<4 nA). When an array of ultramicroelectrodes was positioned with optimal spacing, these BDD sensors showed a sigmoidal diffusion behavior. They also demonstrated high accuracy with linear dose dependence for quantification of cadmium in a certified reference river water sample from the U.S. National Institute of Standards and Technology (NIST) as well as in a human urine sample spiked with 0.25-1 ppb cadmium.