Selective electrochemical determination of cysteine with a cyclotricatechylene modified carbon electrode.

We report the selective electrochemical detection of cysteine in the presence of homocysteine and glutathione with the use of an electrode modified with cyclotricatechylene (CTC). A carbon electrode was first modified with cyclotriveratrylene (CTV) and then electrochemically converted into CTC. Using cyclic voltammetry, the redox activity of CTC was investigated along with its electrochemical response to cysteine and the closely related compounds, glutathione and homocysteine which are commonly found in biological media alongside cysteine. The selective detection of cysteine was achieved with the use of the electrocatalytic oxidation reaction and exploiting the different rates of reaction of each thiol with the oxidized CTC via variable scan rate studies. The analytical parameters consisting of sensitivity, range of linear detection, and limit of detection were determined for selective cysteine detection in phosphate buffer solution and tissue culture media where the sensitivity of the system is ca. 0.023 µA µM(-1) and ca. 0.031 µA µM(-1) with a limit of detection of ca. 0.6 µM and ca. 0.9 µM for buffer solution and tissue culture media respectively. Practical assessment of this analytical method was carried out in mixed solutions containing a combination of cysteine, homocysteine and glutathione in both media. The determined results agree well with the added cysteine content. This work presents a novel way of utilizing CTC into detecting cysteine, and is well-suited for bio-marker sensing.

The use of screen-printed electrodes in a proof of concept electrochemical estimation of homocysteine and glutathione in the presence of cysteine using catechol.

Screen printed electrodes were employed in a proof of concept determination of homocysteine and glutathione using electrochemically oxidized catechol via a 1,4-Michael addition reaction in the absence and presence of cysteine, and each other. Using cyclic voltammetry, the Michael reaction introduces a new adduct peak which is analytically useful in detecting thiols. The proposed procedure relies on the different rates of reaction of glutathione and homocysteine with oxidized catechol so that at fast voltage scan rates only homocysteine is detected in cyclic voltammetry. At slower scan rates, both glutathione and homocysteine are detected. The combination of the two sets of data provides quantification for homocysteine and glutathione. The presence of cysteine is shown not to interfere provided sufficient high concentrations of catechol are used. Calibration curves were determined for each homocysteine and glutathione detection; where the sensitivities are 0.019 µA · µM(-1) and 0.0019 µA · µM(-1) and limit of detections are ca. 1.2 µM and 0.11 µM for homocysteine and glutathione, respectively, within the linear range. This work presents results with potential and beneficial use in re-useable and/or disposable point-of-use sensors for biological and medical applications.

Selective Thiol Detection in Authentic Biological Samples with the Use of Screen-printed Electrodes.

A selective voltammetric determination of homocysteine and glutathione was applied to cell tissue culture media and human plasma via a single two-step method. The two-step method relies on the 1,4-Michael addition reaction between electro-oxidized catechol and the target thiol. Furthermore, the procedure relies on the differing reaction kinetics of the ortho-quinone with various thiol species giving different responses as a function of the scan rate. At faster scan rates homocysteine is only detected, while at slower scan rates the adduct signal reflects both homocysteine and glutathione. As a result, the quantification of both homocysteine and glutathione can be determined with a combination of both sets of data. The previous proof-of-concept (P. T. Lee, D. Lowinsohn, and R. G. Compton, Sensors, 2014, 14, 10395), is applied to the quantification of thiols in both tissue culture media and human plasma alone. Analytical parameters were determined for both homocysteine and glutathione in the respective media and the linear range. The sensitivities in tissue culture media are ca. 3 nA µM(-1) and ca. 1 nA µM(-1) and the limits of detections are ca. 2 µM and ca. 1 µM for homocysteine and glutathione, respectively. In human plasma, the sensitivities were determined to be 94 and 39 nA µM(-1), and the limit of detections are ca. 0.8 µM and ca. 0.8 µM for homocysteine and glutathione, respectively.

Anodic stripping voltammetry of silver nanoparticles: aggregation leads to incomplete stripping.

The influence of nanoparticle aggregation on anodic stripping voltammetry is reported. Dopamine-capped silver nanoparticles were chosen as a model system, and melamine was used to induce aggregation in the nanoparticles. Through the anodic stripping of the silver nanoparticles that were aggregated to different extents, it was found that the peak area of the oxidative signal corresponding to the stripping of silver to silver(I) ions decreases with increasing aggregation. Aggregation causes incomplete stripping of the silver nanoparticles. Two possible mechanisms of 'partial oxidation' and 'inactivation' of the nanoparticles are proposed to account for this finding. Aggregation effects must be considered when anodic stripping voltammetry is used for nanoparticle detection and quantification. Hence, drop casting, which is known to lead to aggregation, is not encouraged for preparing electrodes for analytical purposes.

Tailoring pigment dispersants with polyisobutylene twin-tail structures for electrowetting display application.

We have designed a class of highly hydrophobic dispersants for finely dispersing carbon black and organic pigment nanoparticles in apolar mediums. The synthesis involved the use of polyisobutylene-g-succinic anhydride (PIB-SA) and judiciously selected amines by amidation and imidation. The structures were characterized by infrared spectroscopy for anhydride functionalities in the starting materials and amide/imide linkages in the products. These polymeric forms of dispersants were structurally varied with respects to their PIB molecular weight, twin-tails, and linkages. Their relative performance for dispersing six different pigments in decane was evaluated against solution homogeneity, viscosity, stability, and particle size. The fine dispersion was achieved at particle sizes of ca. 100 nm, with the viscosity as low as 2-3 cP. The measurement of zeta potentials, which varied from -39.8 to -5.1 mV with pigment addition, revealed a strong surface-charge interaction between pigment and PIB dispersant molecules. Examination by TEM (transmission electronic microscope) showed the homogeneous dispersion of the primary structures of pigment particles at ca. 20 nm in diameter. The polymeric dispersants with different PIB tails and imide functionalities could be tailored for pigment stability in the oil phase, which is potentially suitable for the electrowetting devices.

Facile fabrication of robust superhydrophobic epoxy film with polyamine dispersed carbon nanotubes.

Nanocomposite films of superhydrophobic surface are fabricated from the dispersion of unmodified carbon nanotubes (CNTs) and hydrophobic poly(isobutylene)-amine (PIB-amine). The PIB-amine prepared from the amidation of poly(isobutylene)-succinic anhydride and poly(oxypropylene)-amines is essential for dispersing the originally entangled CNTs into the debundled CNTs as observed by TEM. A robust CNTs/epoxy nanocomposite film with high dimensional stability is made by subsequent curing with epoxy resin. The self-standing film exhibits a superhydrophobic property, with water droplet contact angle > 152° due to the CNTs controlled alignment on the surface forming micrometer-size plateaus, as observed by SEM. The preparation of PIB-amine/CNTs dispersion and subsequently curing into a superhydrophobic CNTs/epoxy film is relatively simple and can potentially be applied to large surface coating.

First fabrication of electrowetting display by using pigment-in-oil driving pixels.

We report the first fabrication of pigment particle-based electrowetting display (EWD) by using the requisite poly(isobutylene)-imide (PIB-imide) for effectively dispersing insoluble colorant in decane/water system. The series of PIB-imide dispersants were prepared from the amidation/imidation of PIB-succinic anhydride with different hydrophobic lengths and a suitable amine. The structurally tailored dispersants by adopting the highly hydrophobic PIB tails allows the formation of homogeneous dispersion of nanosized pigment particles in decane and clearly separated from water. The pigment dispersion at particle size of ca. 100 nm and a low viscosity of 2-3 cps was obtained and fabricated into an EWD device which was operated at a driving voltage of 15-20 V in achieving a maximum aperture ratio of 80%. With the advantage of both fast response time and vivid color, the pigment-based EWD, as shown in the video, stands out as a promising new option for future transparent display and serves as a critical foundation for the next-generation advanced display applications.