Lab-on-CMOS integration of microfluidics and electrochemical sensors

This paper introduces a CMOS-microfluidics integration scheme for electrochemical microsystems. A CMOS chip was embedded into a micro-machined silicon carrier. By leveling the CMOS chip and carrier surface to within 100 nm, an expanded obstacle-free surface suitable for photolithography was achieved. Thin film metal planar interconnects were microfabricated to bridge CMOS pads to the perimeter of the carrier, leaving a flat and smooth surface for integrating microfluidic structures. A model device containing SU-8 microfluidic mixers and detection channels crossing over microelectrodes on a CMOS integrated circuit was constructed using the chip-carrier assembly scheme. Functional integrity of microfluidic structures and on-CMOS electrodes was verified by a simultaneous sample dilution and electrochemical detection experiment within multi-channel microfluidics. This lab-on-CMOS integration process is capable of high packing density, is suitable for wafer-level batch production, and opens new opportunities to combine the performance benefits of on-CMOS sensors with lab-on-chip platforms.

Immobilized redox mediators for electrochemical NAD(P)+ regeneration

The applicability of dissolved redox mediators for NAD(P)(+) regeneration has been demonstrated several times. Nevertheless, the use of mediators in solutions for sensor applications is not a very convenient strategy since the analysis is not reagentless and long stabilization times occur. The most important drawbacks of dissolved mediators in biocatalytic applications are interferences during product purification, limited reusability of the mediators, and their cost-intensive elimination from wastewater. Therefore, the use of immobilized mediators has both economic and ecological advantages. This work critically reviews the current state-of-art of immobilized redox mediators for electrochemical NAD(P)(+) regeneration. Various surface modification techniques, such as adsorption polymerization and covalent linkage, as well as the corresponding NAD(P)(+) regeneration rates and the operational stability of the immobilized mediator films, will be discussed. By comparison with other existing regeneration systems, the technical potential and future perspectives of biocatalytic redox reactions based on electrochemically fed immobilized mediators will be assessed.

Electropolymerized poly(Toluidine blue)-modified carbon felt for highly sensitive amperometric determination of NADH in flow injection analysis

Poly(pheniothiazine) films were prepared on a porous carbon felt (CF) electrode surface by an electrooxidative polymerization of three phenothiazine derivatives (i.e.,Tthionine (TN), Toluidine Blue (TB) and Methylene Blue (MB)) from 0.1 mol/L phosphate buffer solution (pH 7.0). Among the three phenothiazies, the poly(TB) film-modified CF exhibited an excellent electrocatalytic activity for the oxidation of nicotinamide adenine dinucleotide reduced form (NADH) at +0.2 V vs. Ag/AgCl. The poly(TB) film-modified CF was successfully used as working electrode unit of highly sensitive amperometric flow-through detector for NADH. The peak currents (peak heights) were almost unchanged, irrespective of a carrier flow rate ranging from 2.0 to 4.1 mL/min, resulting in the measurement of NADH (ca. 30 samples/hr) at 4.1 mL/min. The peak current responses of NADH showed linear relationship over the concentration range from 1 to 30 micromol/L (sensitivity: 0.318 microA/(micromol/L); correlation coefficient: 0.997). The lower detection limit was found to be 0.3 micromol/L (S/N = 3).

Guiding molecules with electrostatic forces in surface enhanced Raman spectroscopy

In surface enhanced raman scattering (SERS) the problem of drawing molecules to the places where surface plasmon resonance enhancements will produce signals is one of the most basic ones, and the initial obstacle to every application of the effect. We explore the possibility of using electrostatic forces as a means to "guide" charged molecules in solution toward SERS active substrates. We also show explicitly the possibility of selectively enhancing different types of dyes according to their charge, and we discuss briefly possible extensions for other applications where "electrostatic guiding" could be an option.

Flow-Injection Analysis and Voltammetric Detection of NADH with a Poly-Toluidine Blue Modified Electrode

Poly-Toluidine Blue film was prepared by electrooxidative polymerization at a glassy carbon electrode in a phosphate buffer solution. The resulting chemically modified electrode (CME) exhibited excellent electrocatalysis toward the oxidation of reduced nicotinamide coenzyme (NADH) with over a 450 mV decrease of the overpotential compared with that at a bare glassy carbon electrode. Two electrochemical determinations of NADH, cyclic voltammetry and flow injection analysis, were established based on the electrocatalytical performance of the resulting modified electrode. Under an identical determinate condition, the voltammetric detection for NADH gave a detection limit of 3.3 micromol L(-1) with a linear concentration range of 9.1 micromol L(-1) to 1.8 mmol L(-1). As a detector in a flow-injection system, the CME gave a detection limit of 0.1 micromol L(-1) for NADH with a linear concentration range of 1.0 micromol L(-1) to 3.2 mmol L(-1). Obviously, flow-injection analysis is superior to voltammetric detection in NADH determination for its lower detection limit and wider detectable linear range.

A binderless, bulk-modified, renewable surface amperometric sensor for NADH and ethanol

Graphite particles are exfoliated and subsequently functionalized with toluidine blue. The resulting covalently modified graphite particles are restacked without any binder to form a surface-renewable, bulk-modified electrode. Electrocatalytic oxidation of NADH and its application in the amperometric biosensing of ethanol using alcohol dehydrogenase enzyme have been demonstrated with this material.

Electrocatalysis of NADH oxidation with an electropolymerized film of 1,4-bis(3,4-dihydroxyphenyl)-2,3-dimethylbutane

The oxidation of 1,4-bis(3,4-dihydroxyphenyl)-2,3-dimethylbutane, known also as nordihydroguaiaretic acid, on a glassy carbon electrode anodically pretreated in KCl solution gives rise to a stabile redox-active polymer containing the o-quinone moiety. The redox response of the modified electrode is typical for a surface-immobilized species. The modifier thickness can be easy controlled by a number of potential cycles applied during electropolymerization, and a surface coverage up to 1.1 x 10(-9) mol cm(-2) can be achieved. The film exhibits catalytic activity toward NADH oxidation. Characteristic kinetic constants for the mediated oxidation of NADH were derived from rotating disk experiments performed in phosphate or Tris/acetate buffers. The effects of film thickness, solution pH, and the presence of Mg2+ cation on the catalytic efficiency of the modified electrode were discussed and compared with literature data concerning related systems.