A novel method for the rapid modification of fullerene for subsequent enzyme attachment to create a potentiometric biosensor is presented. Urease was immobilized onto the modified fullerene nanomaterial. The modified fullerene-immobilized urease (C₆₀-urease) bioconjugate has been confirmed to catalyze the hydrolysis of urea in solution. The biomaterial was then deposited on a screen-printed electrode containing a non-plasticized poly(n-butyl acrylate) (PnBA) membrane entrapped with a hydrogen ionophore. This pH-selective membrane is intended to function as a potentiometric urea biosensor with the deposition of C₆₀-urease on the PnBA membrane. Various parameters for fullerene modification and urease immobilization were investigated. The optimal pH and concentration of the phosphate buffer for the urea biosensor were 7.0 and 0.5 mM, respectively. The linear response range of the biosensor was from 2.31 × 10⁻³ M to 8.28 × 10⁻⁵ M. The biosensor’s sensitivity was 59.67 ± 0.91 mV/decade, which is close to the theoretical value. Common cations such as Na⁺, K⁺, Ca²⁺, Mg²⁺ and NH₄⁺ showed no obvious interference with the urea biosensor’s response. The use of a fullerene-urease bio-conjugate and an acrylic membrane with good adhesion prevented the leaching of urease enzyme and thus increased the stability of the urea biosensor for up to 140 days.

A developed colorimetric pH sensor film based on edible materials for real-time monitoring of food freshness is described. The mixed natural dyes from edible plants Clitoria sp and Brassica sp were extracted and incorporated into ι-carrageenan film as a colorimetric pH sensor film for monitoring food spoilage and its freshness. The color changes of the developed colorimetric sensor film were measured with chromametry and UV-vis spectroscopy, respectively. Experimental results show that colorimetric pH sensor film demonstrated statistically significant differences (p < 0.05) between CIE-L*a*b* coordinates color system indicated that the developed colorimetric sensor film was able to give a gradual change in color over a wide pH range. The color of the colorimetric sensor film also changes discretely and linearly with factors that contribute to food spoilage using shrimp and durian samples. Moreover, the developed colorimetric pH sensor film has the potential to be used as a safe, non-destructive testing and also a flexibly visual method for direct assessment of food freshness indicator during storage.

A novel and simple optical biosensor to detect triglycerides (TGs) has been successfully constructed by using pectin hydrogel membrane as the indicator pH and chromoionophore ETH 5294 (CI), with lipase as the catalyst. The enzymatic working system against TGs releasing H⁺ ions will affect the color absorbance of CI. The characterization results show that a TG biosensor has the optimum condition and sensitivity at the phosphate buffer concentration of 50 mM, pH 7, and enzyme loading of 60 μg. The biosensor works at the tripalmitin (TP) concentration range of 100–400 mg/dL. With the sensitivity of 0.001 (∆A/(mg/dL)), the biosensor response reaches stability after five minutes, and the limit of detection (LOD) of the TG optical biosensor is 15 mg/dL. Relative standard deviation (RSD) in a reproducibility test was 2.5%, with a 15-day lifespan.

Whole cell biosensors always face the challenge of low stability of biological components and short storage life. This paper reports the effects of poly(2-hydroxyethyl methacrylate) (pHEMA) immobilization on a whole cell fluorescence biosensor for the detection of heavy metals (Cu, Pb, Cd), and pesticides (dichlorophenoxyacetic acid (2,4-D), and chlorpyrifos). The biosensor was produced by entrapping the cyanobacterium Anabaena torulosa on a cellulose membrane, followed by applying a layer of pHEMA, and attaching it to a well. The well was then fixed to an optical probe which was connected to a fluorescence spectrophotometer and an electronic reader. The optimization of the biosensor using several factors such as amount of HEMA and drying temperature were undertaken. The detection limits of biosensor without pHEMA for Cu, Cd, Pb, 2,4-D and chlorpyrifos were 1.195, 0.027, 0.0100, 0.025 and 0.025 µg/L respectively. The presence of pHEMA increased the limits of detection to 1.410, 0.250, 0.500, 0.235 and 0.117 µg/L respectively. pHEMA is known to enhance the reproducibility of the biosensor with average relative standard deviation (RSD) of ±1.76% for all the pollutants tested, 48% better than the biosensor without pHEMA (RSD = ±3.73%). In storability test with Cu 5 µg/L, the biosensor with pHEMA performed 11.5% better than the test without pHEMA on day-10 and 5.2% better on day-25. pHEMA is therefore a good candidate to be used in whole cell biosensors as it increases reproducibility and enhances biosensor storability.

A new silica-gel nanospheres (SiO₂NPs) composition was formulated, followed by biochemical surface functionalization to examine its potential in urea biosensor development. The SiO₂NPs were basically synthesized based on sol–gel chemistry using a modified Stober method. The SiO₂NPs surfaces were modified with amine (-NH₂) functional groups for urease immobilization in the presence of glutaric acid (GA) cross-linker. The chromoionophore pH-sensitive dye ETH 5294 was physically adsorbed on the functionalized SiO₂NPs as pH transducer. The immobilized urease determined urea concentration reflectometrically based on the colour change of the immobilized chromoionophore as a result of the enzymatic hydrolysis of urea. The pH changes on the biosensor due to the catalytic enzyme reaction of immobilized urease were found to correlate with the urea concentrations over a linear response range of 50–500 mM (R² = 0.96) with a detection limit of 10 mM urea. The biosensor response time was 9 min with reproducibility of less than 10% relative standard deviation (RSD). This optical urea biosensor did not show interferences by Na⁺, K⁺, Mg²⁺ and NH₄⁺ ions. The biosensor performance has been validated using urine samples in comparison with a non-enzymatic method based on the use of p-dimethylaminobenzaldehyde (DMAB) reagent and demonstrated a good correlation between the two different methods (R² = 0.996 and regression slope of 1.0307). The SiO₂NPs-based reflectometric urea biosensor showed improved dynamic linear response range when compared to other nanoparticle-based optical urea biosensors.

Molecularly imprinted polymers (MIPs) are generally a more stable material for sensing application. The high selectivity and sensitivity of MIPs for sensors can be achieved if the template molecule is imprinted in the polymer and this makes them an ideal alternative as a recognition element for sensors. A new electrochemical sensor based on molecularly imprinted polymeric microspheres (MIPs) and multi-walled carbon nanotube/gold nanoparticle (MIP–MWCNT–AuNP) modified carbon screen-printed electrodes (SPEs) for the rapid detection of 17β-estradiol (E) hormone in serum samples has been successfully developed. Hydrophobic MIPs were synthesized using photopolymerization in emulsion form. The multi-walled carbon nanotube grafted with gold nanoparticles was firstly deposited onto a carbon screen-printed electrode for the purpose of accelerating electron transfer to the surface of the electrode. The MIP microspheres specific to the 17β-estradiol hormone, prepared via a facile photopolymerization technique, were coated onto the MWCNT–AuNP modified SPE. The presence of 17β-estradiol in biological samples could be detected with the sensor via absorption of 17β-estradiol into the deposited MIPs and this was monitored by differential pulse voltammetry (DPV) at 0.6 V for the reduction of 17β-estradiol. Under optimal conditions, the sensor could detect the concentrations of 17β-estradiol from 1.0 × 10⁻¹⁵ to 1.0 × 10⁻⁶ M (R² = 0.9921), with a detection limit of 2.5 × 10⁻¹⁶ M. The sensor based on MIP microspheres and the MWCNT–AuNP modified electrode demonstrated a stability of 55 days with good reproducibility (RSD < 5%, n = 5) and regenerability (RSD < 4%, n = 5). Using this sensor, the gender of the arowana fish determined via the level of 17β-estradiol using fish serum samples demonstrated good agreement with a conventional test kit based on the immuno-assay method.