Xiao Jun Wu

An optical glucose biosensor with eggshell membrane as an enzyme immobilisation platform

An optical glucose biosensor using an enzyme-immobilised eggshell membrane and oxygen-sensitive optode membrane for glucose determination has been developed. Glucose oxidase was covalently immobilised on an eggshell membrane with subsequent covering of the surface with an oxygen-sensitive optode membrane. The oxygen-sensitive membrane was constructed from tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ditetrakis(4-chlorophenyl) borate adsorbed on silica gel particles and dispersed in a silicone rubber film. The glucose biosensor was built by positioning both enzyme-immobilised eggshell and oxygen-sensitive optode membranes in a laboratory-made flow cell. The detection scheme was based on the depletion of dissolved oxygen content upon exposure to glucose solution and the rate change in fluorescence intensity of the oxygen-sensitive membrane was monitored and related to the glucose concentration. The effect of dissolved oxygen content, pH, temperature, phosphate buffer concentration and potential interferents has been studied in detail. The glucose biosensor demonstrated a long shelf-life compared with an enzyme-immobilised cellulose membrane. It has been successfully applied to the determination of glucose content in a beverage sample.

A glucose biosensor with enzyme-entrapped sol–gel and an oxygen-sensitive optode membrane

An optical biosensor for the continuous determination of glucose in beverages based on the canalisation of glucose oxidase into a sol–gel is presented. The enzyme was entrapped within a glass matrix by the sol–gel method. The matrix was ground to a powder form and packed into a laboratory-made flow cell. This minireactor was positioned in a spectrofluorimeter connected to a continuous sample flow system. An oxygen-sensitive optode membrane was fabricated from tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) didodecyl sulfate adsorbed on silica gel particles and entrapped in a silicone-rubber film. The membrane was situated against the wall of the flow cell to sense the depletion of oxygen content upon exposure to glucose. The change of the luminescence intensity of the optode membrane can be related to glucose concentration. The effects of temperature and pH on the response of the biosensor were investigated. Storage, stability and repeatability of the biosensor were also studied in detail. The analytical range of the biosensor was from 0.06 to 30 mmol dm−3 glucose and the time taken to reach a steady signal in a flowing solution was 5–8 min. The detection limit was found to be 6 μmol dm−3. Common matrix interferents such as fructose, galactose, lactose, raffinose, rhamnose, stachyose, sucrose and other components in beverage samples showed no interference. The glucose biosensor has been successfully applied to the determination of glucose contents of beverage samples.

Optode Membrane for Determination of Nicotine via Generation of Its Bromoethane Derivative.

A plasticized poly(vinyl chloride) optode membrane incorporated with a valinomycin ionophore, a H(+)-selective chromoionophore (ETH 5294), and a lipophilic potassium tetrakis(4-chlorophenyl)borate was used as a reversible sensing device for the indirect optical determination of nicotine. Nicotine was extracted from a tobacco product (1-5 g) and converted to its bromoethane derivative (NBD(+)Br(-)) by reacting with a solution of bromoethane in ethanol. NBD(+)Br(-) in a solution of 0.05 M boric acid-Borax buffer and 0.2 mM Triton X-100 was extracted into the bulk of the membrane and subsequently caused changes in optical absorption of the sensing layer. The response slope, dynamic working range, detection limit, sensitivity, selectivity, effects of buffer solution and neutral surfactant Triton X-100, and lifetime were discussed in detail. The response was pH dependent. At pH 8.5, the detection range was extended from 0.4 μM to 1 mM. Typical response times (t(95)) of the samples were 2-4 min. The optode method was successfully used to detect nicotine in a tobacco sample from the market (average content 0.720%; RSD 0.044%; n = 11). The interference of K(+) on the optode method can be prevented by the pre-extraction procedure. Malic acid and citrate showed no interferences. The recovery of nicotine as NBD(+) was 84-119% in the range 0.035-5% nicotine. The result was satisfactory compared with an AOAC UV standard method.

Optosensing of cationic surfactants based on host–guest chemistry

An optode membrane for cationic surfactants is presented. Plasticized poly(vinyl chloride) (PVC) membranes incorporating the hexaester of calix[6]arene and the neutral H+-selective chromo-ionophore (ETH5294) have proved to be excellent reversible sensing devices for cationic surfactants as exemplified by cetyltrimethylammonium bromide (CTMAB). Based on host–guest chemistry, the guest molecule CTMAB was extracted into the PVC membrane, forming a host–guest adduct with the host molecule calix[6]arene ionophore and causing a concomitant release of a proton from the protonated ETH5294 into the solution. Upon deprotonation, ETH5294 undergoes a color change which can be used as a means for the quantitation of CTMAB. Unlike small inorganic cations, such as K+ and Na+, the experimental results show that the membrane response depends to a great extent on the migration process of CTMAB from the bulk of the solution to the membrane interface. A modified theoretical equation describing the membrane response on the CTMAB concentrations has been derived and shown to be in conformity with the experimental results.

Nicotine derivative optode membrane with nonactin as ionophore

Plasticised poly(vinyl chloride) optode membrane incorporated with nonactin as ionophore, ETH 5294 as H(+)-selective chromoionophore and potassium tetrakis(4-chlorophenyl)borate as anionic site, was used as a reversible sensing device for indirect determination of nicotine. Nicotine was extracted from cigarette samples and converted to its bromoethane derivative (ND(+)Br(-)) by reacting with a solution of bromoethane in ethanol. ND(+)Br(-), in a solution of Tris (hydroxymethyl)aminomethane-HCl buffer, was extracted into the bulk of the optode membrane and subsequently caused changes in optical absorption of the sensing layer. The pH effect, response mechanisms, response behaviour, response time, dynamic working range, detection limit, sensitivity and selectivity were discussed in detail.

Optodes based on a calixarene ester for the determination of aldehydes via in situ generation of the Girard’s reagent P derivative

Optode membranes for the general determination of aldehydes by visible spectroscopy were developed. A plasticized poly(vinyl chloride) membrane containing a calixarene ester which is used as a host molecule, a hydrogen ion-selective chromo-ionophore (ETH5294) and potassium tetrakis(4-chlorophenyl)borate (KTpClPB) responds reversibly to a guest molecule which is generated in situ from an aldehyde and a Girard’s reagent P. The sensitivity of the optode can be modulated by changing the concentration of the calixarene ionophore in the membranes and the pH of the measuring solution. The proposed method shows a good correlation with the theoretically derived equation in the range 4 × 10–5–0.2 mol l–1 butyraldehyde with an error of <6% and with a detection limit of 5 × 10–7 mol l–1. Typical response times (t95) for the samples are 5 min.

Optical Enzyme-Based Glucose Biosensors

The development of glucose enzyme biosensor and its associated techniques are succinct reviewed, in the framework of optical transducer. The coverage of this review mainly includes a concise discussion on the characteristics of common optical transducer, enzyme immobilization techniques, enzyme stabilization and mass transfer efficiency, followed by a general description of format and performance of glucose biosensors, and finally a brief commentary on analytical features and applications. Very recent achievements of our group in the technique of glucose biosensor construction, particularly in the use of mixed water-miscible organic solvents and micelle are also covered in this review.